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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Much better, thank you. Couple of small changes, and DR test should be good to go.

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