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

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

> We'd need to see what @alexey-bataev has to say about renaming the OMP assume attribute. I imagine that'll end up causing a headache for his users.

Thanks for the heads up. I don't think renaming to openmp_assume works for OpenMP. We need to support `#pragma omp assume`, `[[omp::assume]]` and `[[using omp : assume]]` as defined by the standard.

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