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

[Stephan Bergmann via cfe-commits]

stbergmann wrote:

I think the discourse thread [Clang is overly aggressive when evaluating constexpr functions](https://discourse.llvm.org/t/clang-is-overly-aggressive-when-evaluating-constexpr-functions/67006) is relevant here.

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