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

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

> `[[clang::assume]]` or `__attribute__((assume))` are the OpenMP one

OpenMP requires only [[omp::assume]], neither [[clang::assume]] nor __attribute__((assume)) are OpenMP requirement.

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