[clang] [Clang] Accept auto parameters pre-C++20 as an extension (PR #200670)

[Aaron Ballman via cfe-commits]

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

Documentation CI failures are unrelated; changes LGTM but for the future, please use git-clang-format to format only the parts of your PR which are being changed (there's quite a bit of unrelated changes which make review harder).

https://github.com/llvm/llvm-project/pull/200670