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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Other than the one diagnostic concern, the changes LGTM and I think are fine for Clang 18.

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