[clang] Add 'enum_select' diagnostic selection to clang. (PR #122505)

[Aaron Ballman via cfe-commits]

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

LGTM with a minor nit on the documentation, but I think this is a great new feature for the diagnostics engine. Thank you for it!

btw, if you happen to have a list of diagnostics that could benefit from switching to use it, that sure seems like Good First Issue territory.

https://github.com/llvm/llvm-project/pull/122505