[clang] [libcxx] [SemaCXX] Implement CWG2137 (list-initialization from objects of the same type) (PR #77768)

[Mark de Wever via cfe-commits]

https://github.com/mordante approved this pull request.

Thanks @cor3ntin. The patch still looks fine from libc++'s PoV.

https://github.com/llvm/llvm-project/pull/77768