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

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Sirraide wrote:

> I think the confusion/conflict between this and clang::assume needs to be figured out. These two should just be, as close as possible, spellings of the same thing.

Well, apparently, `clang::assume` always takes a string and is meant for OMP assumptions, so one solution would be to just apply `clang::assume` semantics iff the argument is a string literal since string literals would always be converted to `true` anyway.

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