[clang] [clang] Add covariance tests that make sure we return an error when return value is different in pointer / lvalue ref / rvalue ref (PR #112853)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

I left suggestions for one of the errors you're expecting. You should apply those to the rest of expected directives.

https://github.com/llvm/llvm-project/pull/112853