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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

@Endilll , are you ok with the changes?
BTW, do we want this for 18, or it is better to wait and merge after cutoff?

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