MachOPlatform::BootstrapInfo::Mutex was meant to be used to synchronize access
to the MachOPlatform::BootstrapInfo::ActiveGraphs member, but the latter was
also modified under the MachOPlatform::PlatformMutex (in
MachOPlatform::MachOPlatformPlugin::modifyPassConfig), leading to a data race.

There have been external reports (rdar://151041549) of deadlocks on the
MachOPlatform::BootstrapInfo::CV condition variable that are consistent with
corruption of the ActiveGraphs member (though alternative explanations for
the reported behavior exist, and it has been too rare in practice to verify).

This patch removes the misused MachOPlatform::BootstrapInfo::Mutex member and
synchronizes all accesses to ActiveGraphs using MachOPlatform::PlatformMutex
instead. Since ActiveGraphs is only used during bootstrap the performance
impact of this should be negligible.

rdar://151041549 - possible fix.

MachOPlatform synthesizes Mach headers for JITDylibs (see ef314d39b92). This
commit adds support for adding LC_LOAD_WEAK_DYLIB commands to these synthesized
headers (LC_LOAD_DYLIB was already supported previously).

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

`Sym.getSection()` is equivalent to `Sym.getBlock().getSection()`.

Some eh-frame records are CIEs, which don't point to functions. We need to skip
these records. This patch reuses EHFrameCFIBlockInspector to identify function
targets, rather than a custom loop. Any performance impact will be minimal, and
essentially irrelevant once compact-unwind support re-lands (since at that
point we'll discard most eh-frame records).

For unwind-info sections: don't assume one block per record: the unwind-info
section packs all records into a single block.

Unwind-info records only have one keep-alive edge to their target function, but
eh-frame records may have multiple edges (to the CIE, function, personality, and
lsda). We need to identify the target-function edge differently for each section
type.

a1ff2d18466 should have disambiguated MachOCompactUnwindInfoSectionName to
MachOUnwindInfoSectionName, given how it's used in MachOPlatform and its
value.

A MachOCompactUnwindSectionName variable with an appropriate value will be
added in an upcoming patch to re-enable compact-unwind support in JITLink.