[clang] [clang] Add clang::preferred_type attribute for bitfields (PR #69104)

[Aaron Ballman via cfe-commits]

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

These changes LGTM, but please give @erichkeane a chance to chime in given his previous feedback.

https://github.com/llvm/llvm-project/pull/69104