[clang] [Clang] Permit both `gnu` and `clang` prefixes on attributes (PR #125796)

[Aaron Ballman via cfe-commits]

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

LGTM, but please be sure to add a release note to `clang/docs/ReleaseNotes.rst` so users know about the changes as well. Also, before landing, please change the summary -- this isn't allowing both `clang` and `gnu` on *all* attributes, just on `no_sanitize` (I was pretty confused at first!).

https://github.com/llvm/llvm-project/pull/125796