[Remarks] Add an LLVM-bitstream-based remark serializer
Add a new serializer, using a binary format based on the LLVM bitstream
This format provides a way to serialize the remarks in two modes:
- Separate mode: the metadata is separate from the remark entries.
- Standalone mode: the metadata and the remark entries are in the same
The format contains:
- a meta block: container version, container type, string table,
external file path, remark version
- a remark block: type, remark name, pass name, function name, debug
file, debug line, debug column, hotness, arguments (key, value, debug
file, debug line, debug column)
A string table is required for this format, which will be dumped in the
meta block to be consumed before parsing the remark blocks.
On clang itself, we noticed a size reduction of 13.4x compared to YAML,
and a compile-time reduction of between 1.7% and 3.5% on CTMark.
Differential Revision: https://reviews.llvm.org/D63466