[clang] [Clang][C++23] update constexpr diagnostics for missing return statements per P2448 (PR #94123)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

I wonder if we should always emit an error when a function with non-void return type doesn't have a return statement? In any case it is a serious yet dumb bug that will lead to problems and perhaps some time spent debugging it.
gcc rejects the case for C++23 as well https://godbolt.org/z/8xq8TdEGE . 
cc @AaronBallman 

https://github.com/llvm/llvm-project/pull/94123