[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]

Endilll wrote:

> clang/www/make_cxx_dr_status seems to generate a lot of unrelated changes, so perhaps a separate pull request would make sense if we want to update cxx_dr_status.html.

Ignore other changes; just include the one that updates CWG960 status.
I'll update the rest as an NFC change.

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