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

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Sirraide wrote:

In that case, we could maybe combine some of the approaches—i.e. differentiate the attributes based on appertainment, but also deprecate `[[clang::assume]]`/`__attribute__((assume))` and tell people to use `[[omp::assume]]` instead, if that is possible?

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