Re-enables compact-unwind support in JITLink, which was reverted in b04847b427d
due to buildbot failures.

The underlying cause for the failures on the buildbots was the lack of
compact-unwind registration support on older Darwin OSes. Since the
CompactUnwindManager pass now removes eh-frames by default we were left with
unwind-info that could not be registered. On x86-64, where eh-frame info is
produced by default the solution is to fall back to using eh-frames. On arm64
we simply can't support exceptions on older OSes.

This patch updates the EHFrameRegistrationPlugin to remove the compact-unwind
section (__LD,__compact_unwind) when installed, forcing use of eh-frames when
the EHFrameRegistrationPlugin is used. In LLJIT, the EHFrameRegistrationPlugin
continues to be used for all non-Darwin platform, and will be added on Darwin
platforms when the a CompactUnwindRegistrationPlugin instance can't be created
(e.g. due to missing support for compact-unwind info registration).

The lit.cfg.py script is updated to check whether the host OSes default unwind
info supports JIT registration, allowing tests to be disabled for older Darwin
OSes on arm64.

This reverts commit d6524c8dfa37634257050ca71d16e117b802181c. This
reverts commit b1bd73700a1fb6f450e0f6f9c405a9c8bde2cae7.

This was causing bot failures on Darwin


https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-cmake-RA-incremental/7315/
  Clang.Interpreter.simple-exception.cpp

Clang-Unit.Interpreter/ExceptionTests/_/ClangReplInterpreterExceptionTests/0.1
  LLVM.ExecutionEngine/OrcLazy.minimal-throw-catch.ll

https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-RA/3415/
  ORC-x86_64-darwin.TestCases/Darwin/Generic.exceptions.cpp
  ORC-x86_64-darwin.TestCases/Darwin/x86-64.lljit-ehframe.cpp

This reapplies 4f0325873fa (and follow up patches 26fc07d5d88, a001cc0e6cdc,
c9bc242e387, and fd174f0ff3e), which were reverted in 212cdc9a377 to
investigate bot failures (e.g.
https://lab.llvm.org/buildbot/#/builders/108/builds/8502)

The fix to address the bot failures was landed in d0052ebbe2e. This patch also
restricts construction of the UnwindInfoManager object to Apple platforms (as
it won't be used on other platforms).

This reverts 4f0325873faccfbe171bae4babceb65975ca892e and follow-up patches
(see below) while I investigate some ongoing failures on the buildbots.

---

Revert "[clang-repl] Try to XFAIL testcase on arm32 without affecting arm64
darwin."

This reverts commit fd174f0ff3e793fe96a6663b1488ed159cfe042f.

Revert "[clang-repl] The simple-exception test now passes on arm64-darwin."

This reverts commit c9bc242e387f4a4a3dfcd86561f3ec0ca8a72d62.

Revert "[ORC] Destroy defunct MaterializationUnits outside the session lock."

This reverts commit a001cc0e6cdcfa672b8aff9ce6d14782bb96356a.

Revert "[ORC] Add explicit narrowing casts to fix build errors."

This reverts commit 26fc07d5d88760ad659599184fd10181287d2d9e.

Revert "[ORC] Enable JIT support for the compact-unwind frame info format on
Darwin."

This reverts commit 4f0325873faccfbe171bae4babceb65975ca892e.

For Darwin/arm64 (including Apple Silicon Macs) this will enable exception
handling and stack unwinding in JIT'd code.

Darwin supports two unwind-info formats: DWARF eh-frames and compact-unwind. On
Darwin/x86-64 compilers usually produce both by default, and ORC supported
exceptions and unwinding via eh-frames (same as on Linux), discarding the
redundant compact-unwind info. On Darwin/arm64 compilers typically default to
producing compact-unwind only, with DWARF eh-frames as a fallback for functions
that can't be described in compact-unwind. Since ORC did not previously support
the compact-unwind format and eh-frames were not present ORC was unable to
handle exceptions or unwinding by default in Darwin/arm64 JIT'd code.

This patch enables support for the compact-unwind-info format, and contains
three major moving parts:

(1) The JITLink CompactUnwindManager class is responsible for transforming the
    __compact_unwind records produced by the linker into the __unwind_info
    tables that libunwind parses during unwinding. To enable this the
    CompactUnwindManager class provides three JITLink passes: The
    prepareForPrune pass that splits the __compact_unwind section into
    single-record blocks, allowing unused records to be dead-stripped; the
    processAndReserveUnwindInfo pass that reserves space for the final
    __unwind_info section, and the writeUnwindInfo pass that writes the
    __unwind_info section.

(2) The OrcTargetProcess UnwindInfoManager class maintains a table of
    registered JIT'd __unwind_info and __eh_frame sections, and handles
    requests from libunwind for unwind info sections (by registering a callback
    with libunwind's __unw_add_find_dynamic_unwind_sections function).

(3) The Orc UnwindInfoRegistrationPlugin, which scans LinkGraphs for
    __unwind_info and __eh_frame sections to register with the
    UnwindInfoManager.

This commit adds the CompactUnwindManager passes to the default JITLink
pipelines for Darwin/arm64 and Darwin/x86-64, and UnwindInfoManager intances to
the SelfExecutorProcessControl class (when built for apple platforms) and the
llvm-jitlink-executor tool.

The LLJIT class will now create an UnwindInfoRegistrationPlugin when targeting
a process running on Darwin if it detects that an UnwindInfoManager is
available to handle the registrations.

The ORC runtime macho_platform class already supported libunwind callbacks, so
out-of-process execution and unwinding support will work when loading the ORC
runtime.

The llvm-jitlink tool will only support compact-unwind when the orc-runtime is
loaded, as the UnwindInfoRegistrationPlugin requires access to an IR compiler
to load a helper module and llvm-jitlink does not provide an IR compiler.

Identified with misc-include-cleaner.

This reverts commit d941d597837d9e1405086f008c9bd6a71e7263c9.

Differential Revision: https://reviews.llvm.org/D122258

This reverts commit ef501bf85d8c869248e51371f0e74bcec0e7b229.

Previously, omitting unnecessary DWARF unwinds was only done in two
cases:
* For Darwin + aarch64, if no DWARF unwind info is needed for all the
  functions in a TU, then the `__eh_frame` section would be omitted
  entirely. If any one function needed DWARF unwind, then MC would emit
  DWARF unwind entries for all the functions in the TU.
* For watchOS, MC would omit DWARF unwind on a per-function basis, as
  long as compact unwind was available for that function.

This diff makes it so that we omit DWARF unwind on a per-function basis
for Darwin + aarch64 as well. In addition, we introduce the flag
`--emit-dwarf-unwind=` which can toggle between `always`,
`no-compact-unwind` (only emit DWARF when CU cannot be emitted for a
given function), and the target platform `default`.  `no-compact-unwind`
is particularly useful for newer x86_64 platforms: we don't want to omit
DWARF unwind for x86_64 in general due to possible backwards compat
issues, but we should make it possible for people to opt into this
behavior if they are only targeting newer platforms.

**Motivation:** I'm working on adding support for `__eh_frame` to LLD,
but I'm concerned that we would suffer a perf hit. Processing compact
unwind is already expensive, and that's a simpler format than EH frames.
Given that MC currently produces one EH frame entry for every compact
unwind entry, I don't think processing them will be cheap. I tried to do
something clever on LLD's end to drop the unnecessary EH frames at parse
time, but this made the code significantly more complex. So I'm looking
at fixing this at the MC level instead.

**Addendum:** It turns out that there was a latent bug in the X86
backend when `OmitDwarfIfHaveCompactUnwind` is naively enabled, which is
not too surprising given that this combination has not been heretofore
used.

For functions that have unwind info that cannot be encoded with CU, MC
would end up dropping both the compact unwind entry (OK; existing
behavior) as well as the DWARF entries (not OK).  This diff fixes things
so that we emit the DWARF entry, as well as a CU entry with encoding
`UNWIND_X86_MODE_DWARF` -- this basically tells the unwinder to look for
the DWARF entry. I'm not 100% sure the `UNWIND_X86_MODE_DWARF` CU entry
is necessary, this was the simplest fix. ld64 seems to be able to handle
both the absence and presence of this CU entry. Ultimately ld64 (and
LLD) will synthesize `UNWIND_X86_MODE_DWARF` if it is absent, so there
is no impact to the final binary size.

Reviewed By: davide, lhames

Differential Revision: https://reviews.llvm.org/D122258

Most of `MemoryBuffer` interfaces expose a `RequiresNullTerminator` parameter that's being used to:
* determine how to open a file (`mmap` vs `open`),
* assert newly initialized buffer indeed has an implicit null terminator.

This patch adds the paramater to the `SmallVectorMemoryBuffer` constructors, meaning:
* null terminator can now be added to `SmallVector`s that didn't have one before,
* `SmallVectors` that had a null terminator before keep it even after the move.

In line with existing code, the new parameter is defaulted to `true`. This patch makes sure all calls to the `SmallVectorMemoryBuffer` constructor set it to `false` to preserve the current semantics.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D115331