[clang] [Clang][Sema] Implement proposed resolution for CWG2847 (PR #80899)

[Vlad Serebrennikov via cfe-commits]

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

DR testing looks good now. Thank you for bearing with me!
I tried to give this a proper in-depth review, but the only corner case that comes to my mind is conversion functions. Feel free to dismiss it if you don't think it's relevant or interesting.

https://github.com/llvm/llvm-project/pull/80899