[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]

================
@@ -93,6 +93,38 @@ struct B : A {
 } // namespace example2
 } // namespace cwg952
 
+namespace cwg960 { // cwg960: 2.8
----------------
Endilll wrote:

```suggestion
namespace cwg960 { // cwg960: 3.0
```
https://godbolt.org/z/o77989zM7

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