[clang] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

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dyung wrote:

> Nevermind, unreverted since there appear to be newer tests that use this patch.

@amykhuang your unrevert (9bfa506d69b9177ced00b69bf94b28038b063d6d) appears to be an empty commit? Was that intended?

https://github.com/llvm/llvm-project/pull/77753