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

[via cfe-commits]

cor3ntin wrote:

> @cor3ntin , I wonder, since we want to claim it as an extension in earlier language modes, should I actually add an option for this extension and set these flags to this option value, not even language mode? Or is that not worth it?

As long as there is an extension warning I think we don't need a flag (Werror and -pedantic are enough flexibility).
So I guess we should set the `DefaultedDestructorIsConstexpr` to false and only use it for warning? I think that makes sense. I haven't thought of that, sorry.

All of that complicated machinery is now there just for warning, haha (but the warnings are useful)

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