[clang] [Clang] Implement resolution for CWG1835 (PR #92957)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

I added support for unqualified lookup finding multiple declarations in the most recent commits, fixing the crash the currently happens on clang assertions trunk for the following ([godbolt link](https://godbolt.org/z/rEzo76qr5)):
```cpp
inline namespace N
{
    struct A { };
}

struct A { };

template<typename T>
void f(T& t)
{
    t.A::x; // currently causes a crash
}
```

https://github.com/llvm/llvm-project/pull/92957