[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.
> 
> Yeah, the weird thing is I’m not sure we support `[[omp::assume]]` at the moment. We should definitely fix this entire situation, but I’m not sure it needs to be part of this pr.

Can you somehow fix omp::assume at first? Say, in a separate patch?

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