[Clang][Sema] Diagnose class member access expressions naming non-existent...

[Clang][Sema] Diagnose class member access expressions naming non-existent members of the current instantiation prior to instantiation in the absence of dependent base classes (#84050)

Consider the following:
```cpp
template<typename T>
struct A
{
    auto f()
    {
        return this->x;
    }
};
```
Although `A` has no dependent base classes and the lookup context for
`x` is the current instantiation, we currently do not diagnose the
absence of a member `x` until `A<T>::f` is instantiated. This patch
moves the point of diagnosis for such expressions to occur at the point
of definition (i.e. prior to instantiation).