[clang] [clang-tools-extra] Reland "[clang] Don't add documentation comments to the AST if not requested (#206363)" (PR #221605)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

The caching behavior looks right as far as I can tell, and glad this is going to get perf back to where it needs to be.  Wouldn't mind someone else double checking though.

https://github.com/llvm/llvm-project/pull/221605