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

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

I think all the comments made on phab have been applied, and I do believe this implements the DRs (or their intent)

Please give a few days for other folks to look at it before merging.
@zygoloid in particular as he opened CW2311

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