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

[Aaron Ballman via cfe-commits]

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

The implementation looks reasonable to me, and I appreciate the dedicated test coverage for the pragma cases now that we're caching things. LG!

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