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

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Sirraide 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.

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