[clang] [clang-tools-extra] [clang] Don't add documentation comments to the AST if not requested (PR #206363)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think this should come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the improved memory overhead, and we should also add tests showing that we retain or drop the comment AST nodes (probably in `clang/test/AST`).

https://github.com/llvm/llvm-project/pull/206363