[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

[via cfe-commits]

Sirraide wrote:

> Change __attribute__((assume)) (and [[clang::assume]] if we care enough about that one) to be synonyms for [[assume]].

Actually, I think we could do that immediately because OpenMP’s `assume` isn’t valid as a statement attribute anyway iirc. I’d still deprecate all spellings other than `[[omp::assume]]` on functions, though—unless there is some problem with that that I’m missing.

https://github.com/llvm/llvm-project/pull/81014