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

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Sirraide wrote:

> Also note, we need to update the feature-test-macro/has_attribute-expr here.

I’ve already added a check for `__has_cpp_attribute` to one of the tests. `__has_attribute(assume)` already evaluates to `1` even without this pr, likely because of `clang::assume`, but I can add a test for that as well. I’m also not aware of a feature test macro for `assume`? AFAIK it’s just `__has_cpp_attribute`.

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