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

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Sirraide wrote:

We also need to figure out what to do w/ `clang::assume`, because the entire situation with the two being separate attributes is a bit of a mess, as @erichkeane also pointed out. That can probably be resolved separately from the RFC, however—though I’m not that familiar w/ `clang::assume`, so I may be wrong about this.

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