Enables parallelization for the processing of DWO CUs.

When runOnEachFunctionWithUniqueAllocId is invoked with
ForceSequential=true, then the current implementation runs the function
with AllocId==0, which is the Id for the shared, non-unique, default
AnnotationAllocator.

However, the documentation for runOnEachFunctionWithUniqueAllocId
states:
```
/// Perform the work on each BinaryFunction except those that are rejected
/// by SkipPredicate, and create a unique annotation allocator for each
/// task. This should be used whenever the work function creates annotations to
/// allow thread-safe annotation creation.
```

Therefore, even when ForceSequential==true, a unique AllocId should be
used, i.e. different from 0.

In the current upstream BOLT this is presumably not depended on, but it
is needed to reduce memory usage for analyses that use a lot of
memory/annotations. Examples are the pac-ret and stack-clash analyses
that currently have prototype implementations as described in
https://discourse.llvm.org/t/rfc-bolt-based-binary-analysis-tool-to-verify-correctness-of-security-hardening/78148
These analyses use the DataFlowAnalysis framework to sometimes store
quite a lot of information on each MCInst. They run in parallel on each
function. When the dataflow analysis is finished, the annotations on
each MCInst can be removed, hugely saving on memory consumption. The
only annotations that need to remain are those that indicate some
unexpected properties somewhere in the binary.

Fixing this bug enables implementing the deletion of the memory used by
those huge number of DataFlowAnalysis annotations (by invoking
BC.MIB->freeValuesAllocator(AllocatorId)), even when run with
--no-threads. Without this bug fixed, the invocation of
BC.MIB->freeValuesAllocator(AllocatorId) results in also the memory for
all other annotations to be deleted, as AllocatorId is 0.

---------

Co-authored-by: Maksim Panchenko <maks@meta.com>

The base class llvm::ThreadPoolInterface will be renamed
llvm::ThreadPool in a subsequent commit.

This is a breaking change: clients who use to create a ThreadPool must
now create a DefaultThreadPool instead.

Make core BOLT functionality more friendly to being used as a
library instead of in our standalone driver llvm-bolt. To
accomplish this, we augment BinaryContext with journaling streams
that are to be used by most BOLT code whenever something needs to
be logged to the screen. Users of the library can decide if logs
should be printed to a file, no file or to the screen, as
before. To illustrate this, this patch adds a new option
`--log-file` that allows the user to redirect BOLT logging to a
file on disk or completely hide it by using
`--log-file=/dev/null`. Future BOLT code should now use
`BinaryContext::outs()` for printing important messages instead of
`llvm::outs()`. A new test log.test enforces this by verifying that
no strings are print to screen once the `--log-file` option is
used.

In previous patches we also added a new BOLTError class to report
common and fatal errors, so code shouldn't call exit(1) now. To
easily handle problems as before (by quitting with exit(1)),
callers can now use
`BinaryContext::logBOLTErrorsAndQuitOnFatal(Error)` whenever code
needs to deal with BOLT errors. To test this, we have fatal.s
that checks we are correctly quitting and printing a fatal error
to the screen.

Because this is a significant change by itself, not all code was
yet ported. Code from Profiler libs (DataAggregator and friends)
still print errors directly to screen.

Co-authored-by: Rafael Auler <rafaelauler@fb.com>

Test Plan: NFC

This has the following advantages:
- std::shared_timed_mutex is macOS 10.12+ only. llvm::sys::RWMutex
  automatically switches to a different implementation internally
  when targeting older macOS versions.
- bolt only needs std::shared_mutex, not std::shared_timed_mutex.
  llvm::sys::RWMutex automatically uses std::shared_mutex internally
  where available.

std::shared_mutex and RWMutex have the same API, so no code changes
other than types and includes are needed.

Differential Revision: https://reviews.llvm.org/D138423

Summary: Fix comments at the start of source files.

(cherry picked from FBD33274597)

Summary:
Moves source files into separate components, and make explicit
component dependency on each other, so LLVM build system knows how to
build BOLT in BUILD_SHARED_LIBS=ON.

Please use the -c merge.renamelimit=230 git option when rebasing your
work on top of this change.

To achieve this, we create a new library to hold core IR files (most
classes beginning with Binary in their names), a new library to hold
Utils, some command line options shared across both RewriteInstance
and core IR files, a new library called Rewrite to hold most classes
concerned with running top-level functions coordinating the binary
rewriting process, and a new library called Profile to hold classes
dealing with profile reading and writing.

To remove the dependency from BinaryContext into X86-specific classes,
we do some refactoring on the BinaryContext constructor to receive a
reference to the specific backend directly from RewriteInstance. Then,
the dependency on X86 or AArch64-specific classes is transfered to the
Rewrite library. We can't have the Core library depend on targets
because targets depend on Core (which would create a cycle).

Files implementing the entry point of a tool are transferred to the
tools/ folder. All header files are transferred to the include/
folder. The src/ folder was renamed to lib/.

(cherry picked from FBD32746834)