[clang] [Clang] Diagnose defaulted assignment operator with incompatible object parameter (PR #70176)

[Aaron Ballman via cfe-commits]

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

LGTM though something seems to be amiss with precommit CI and so it might be nice to get a clean run of that before landing (I don't insist).

https://github.com/llvm/llvm-project/pull/70176