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

That’s independent of the changes introduced by this pr, from what I can tell at least. So it should be possible to do that irrespective of when this gets merged, yeah.

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