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

[Timm Baeder via cfe-commits]

tbaederr wrote:

FYI: https://llvm-compile-time-tracker.com/compare.php?from=5250ccfa92a4388a288ebccd09eebe8c7587aaac&to=4f9eb0b9e249a7c8667e5fd5ed1f099fc6c65b05&stat=instructions:u

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