[clang] [libcxx] [Clang] Diagnose forming references to nullptr (PR #143667)

[Aaron Ballman via cfe-commits]

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

So long as @philnik777 is okay with the libc++ changes, this LGTM

https://github.com/llvm/llvm-project/pull/143667