[Clang][Sema] access checking of friend declaration should not be delayed (#91430)

attempt to fix https://github.com/llvm/llvm-project/issues/12361
Consider this example:
```cpp
class D {
    class E{
        class F{};
        friend  void foo(D::E::F& q);
        };
    friend  void foo(D::E::F& q);
    };

void foo(D::E::F& q) {}
```
The first friend declaration of foo is correct. After that, the second
friend declaration delayed access checking and set its previous
declaration to be the first one. When doing access checking of `F`(which
is private filed of `E`), we put its canonical declaration(the first
friend declaration) into `EffectiveContext.Functions`. Actually, we are
still checking the first one. This is incorrect due to the delayed
checking.
Creating a new scope to indicate we are parsing a friend declaration and
doing access checking in time.