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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

I checked changes to DR tests, as that's the only part of this PR I'm competent enough for.

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