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

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Sirraide wrote:

Wrt `[[clang::assume]]`/`__attribute__((assume))` applied to statements, those aren’t valid today anyway, so not supporting the former wouldn’t be a breaking change. We could consider supporting the latter though for GCC compatibility (afaik GCC’s `__attribute__((assume))` is pretty much `[[assume]]`).

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