This commit adds operations to orc::MemoryAccess for reading basic types
(uint8_t, uint16_t, uint32_t, uint64_t, pointers, buffers, and strings)
from executor memory.

The InProcessMemoryAccess and EPCGenericMemoryAccess implementations are
updated to support the new operations.

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

Make EPCGenericMemoryAccess the default implementation for the MemoryAccess
object in SimpleRemoteEPC, and add support for the WritePointers operation to
OrcTargetProcess (previously this operation was unimplemented and would have
triggered an error if accessed in a remote-JIT setup).

No testcase yet: This functionality requires cross-process JITing to test (or a
much more elaborate unit-test setup). It can be tested once the new top-level
ORC runtime project lands.

Introduces COFFVCRuntimeBootstrapper that loads/initialize vc runtime libraries. In COFF, we *must* jit-link vc runtime libraries as COFF relocation types have no proper way to deal with out-of-reach data symbols ragardless of linking mode. (even dynamic version msvcrt.lib have tons of static data symbols that must be jit-linked) This class tries to load vc runtime library files from msvc installations with an option to override the path.

There are some complications when dealing with static version of vc runtimes. First, they need static initializers to be ran that requires COFFPlatform support but orc runtime will not be usable before vc runtimes are fully initialized. (as orc runtime will use msvc stl libraries) COFFPlatform that will be introduced in a following up patch will collect static initializers and run them manually in host before boostrapping itself. So, the user will have to do the following.
1. Create COFFPlatform that addes static initializer collecting passes.
2. LoadVCRuntime
3. InitializeVCRuntime
4. COFFPlatform.bootstrap()
Second, the internal crt initialization function had to be reimplemented in orc side. There are other ways of doing this, but this is the simplest implementation that makes platform fully responsible for static initializer. The complication comes from the fact that crt initialization functions (such as acrt_initialize or dllmain_crt_process_attach) actually run all static initializers by traversing from `__xi_a` symbol to `__xi_z`. This requires symbols to be contiguously allocated in sections alphabetically sorted in memory, which is not possible right now and not practical in jit setting. We might ignore emission of `__xi_a` and `__xi_z` symbol and allocate them ourselves, but we have to take extra care after orc runtime boostrap has been done -- as that point orc runtime should be the one running the static initializers.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D130456

These types performed identical roles. Merging them simplifies interoperability
between JITLink and ORC APIs (allowing us to address a few FIXMEs).

This type has been moved up into the llvm::orc::shared namespace.

This type was originally put in the detail:: namespace on the assumption that
few (if any) LLVM source files would need to use it. In practice it has been
needed in many places, and will continue to be needed until/unless
OrcTargetProcess is fully merged into the ORC runtime.

This re-applies e32b1eee6aab52e2b7b75ee15e506b3e7dd30e68, which was reverted in
20675d8f7dab293172266fdde426c2173b5b3997 due to broken unit tests. This patch
includes fixes for the tests.

This reverts commit e32b1eee6aab52e2b7b75ee15e506b3e7dd30e68.

Reverting while I fix some broken unit tests.

SPSExecutorAddr will now be serializable to/from ExecutorAddr, rather than
uint64_t. This improves type safety when working with serialized addresses.

Also updates the SupportFunctionCall to use an ExecutorAddrRange (rather than
a separate ExecutorAddr addr and uint64_t size field), and updates the
tpctypes::*Write data structures to use ExecutorAddr rather than
JITTargetAddress.

This reintroduces "[ORC] Introduce EPCGenericRTDyldMemoryManager."
(bef55a2b47a938ef35cbd7b61a1e5fa74e68c9ed) and "[lli] Add ChildTarget dependence
on OrcTargetProcess library." (7a219d801bf2c3006482cf3cbd3170b3b4ea2e1b) which were
reverted in 99951a56842d8e4cd0706cd17a04f77b5d0f6dd0 due to bot failures.

The root cause of the bot failures should be fixed by "[ORC] Fix uninitialized
variable." (0371049277912afc201da721fa659ecef7ab7fba) and "[ORC] Wait for
handleDisconnect to complete in SimpleRemoteEPC::disconnect."
(320832cc9b7e7fea5fc8afbed75c34c4a43287ba).