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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Yeah, I think this is related to https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0859r0.html and I think this comment from @zygoloid might be related as well: https://github.com/llvm/llvm-project/issues/12223#issuecomment-980903515

It's possible this is valid for Clang to reject because of eager instantiation, but I'm not 100% convinced yet either. CC @erichkeane @hubert-reinterpretcast as well

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