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

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Sirraide wrote:

I agree that renaming it to `openmp_assume` would probably avoid most complications, so if that’s not too big of a deal for the people that use this attribute then I’d probably go with that, otherwise option 3; with how `[[assume]]` works, I don’t particularly see it appertaining to declarations in a sane manner, but you never know—and not giving two unrelated attributes the same name would definitely be a more permanent fix.

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