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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I like this idea/direction.  I've only reviewed the "AreAllDiagnosticsIgnored" stuff (which I actually wrote for a different patch!) but that looks good, as does the rest of the patch.  BUT Aaron should be the one who approves all of this.

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