[libcxx] [lldb] [clang-tools-extra] [libc] [compiler-rt] [llvm] [clang] [flang] [lld] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> So I guess we should set the DefaultedDestructorIsConstexpr to false and only use it for warning?

I'm not sure? Switching all constexpr-related errors to warnings doesn't seem right, even though almost all functions now can be marked constexpr, they still can't be called from constexpr context. In addition, if we keep DefaultedDestructorIsConstexpr always `false` we still will produce _some_ diagnostic for code in https://github.com/llvm/llvm-project/pull/78195#issuecomment-1895950521 and we should not, I suppose?
Sorry if I'm misunderstanding and being slow.

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