[clang] [clang] Add tests for CWG issues about friend declaration matching (PR #106117)

[via cfe-commits]

cor3ntin wrote:

I think both  CWG1900 and CW1477 can be marked as implemented with such a test 
```cpp
namespace N {
struct A {
    friend int f();
};
}
int N::f() { return 0; }
int N::g() { return 0; } 
// expected-error at -1 {{out-of-line definition of 'g' does not match any declaration in namespace 'N'}}```

Resolved by https://eel.is/c++draft/dcl.meaning#general-3.4.sentence-2 added by p1787r6


https://github.com/llvm/llvm-project/pull/106117