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

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Sirraide wrote:

@erichkeane @AaronBallman Do we want to figure out what to do wrt OpenMP’s `assume` attribute before we merge this, or do we want to decide on that after this is merged? Currently, only `[[assume]]` is treated as the standard attribute, and `[[clang::assume]]` or `__attribute__((assume))` are the OpenMP one.

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