This moves the MemoryAccess interface out of the ExecutorProcessControl
class and splits implementation classes InProcessMemoryManager and
SelfExecutorProcessControl out of ExecutorProcessControl.h and into
their own headers.

This include is only used for implementation, so shouldn't be in the public
header.

The find-dynamic-unwind-info callback registration APIs in libunwind
limit the number of callbacks that can be registered. If we use multiple
UnwindInfoManager instances, each with their own own callback function
(as was the case prior to this patch) we can quickly exceed this limit
(see https://github.com/llvm/llvm-project/issues/126611).

This patch updates the UnwindInfoManager class to use a singleton
pattern, with the single instance shared between all LLVM JITs in the
process.

This change does _not_ apply to compact unwind info registered through
the ORC runtime (which currently installs its own callbacks).

As a bonus this change eliminates the need to load an IR "bouncer"
module to supply the unique callback for each instance, so support for
compact-unwind can be extended to the llvm-jitlink tools (which does not
support adding IR).

The system libunwind on older Darwins does not support JIT registration of
compact-unwind. Since the CompactUnwindManager utility discards redundant
eh-frame FDEs by default we need to remove the compact-unwind section first
when targeting older libunwinds in order to preserve eh-frames.

While LLJIT was already doing this as of eae6d6d18bd, MachOPlatform was not.
This was causing buildbot failures in the ORC runtime (e.g. in
https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-RA/3479/).

This patch updates both LLJIT and MachOPlatform to check a bootstrap value,
"darwin-use-ehframes-only", to determine whether to forcibly preserve
eh-frame sections. If this value is present and set to true then compact-unwind
sections will be discarded, causing eh-frames to be preserved. If the value is
absent or set to false then compact-unwind will be used and redundant FDEs in
eh-frames discarded (FDEs that are needed by the compact-unwind section are
always preserved).

rdar://143895614

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.