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

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Sirraide wrote:

And yes, it looks like the current implementation already differentiates the two attributes properly based on appertainment; the only thing left to do would be to investigate if there are any (other) diagnostics that need some improvement.

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