[Clang][Sema] Use the correct lookup context when building overloaded 'operator->' in the current instantiation (#104458)

Currently, clang erroneously rejects the following:
```
struct A
{
    template<typename T>
    void f();
};

template<typename T>
struct B
{
    void g()
    {
        (*this)->template f<int>(); // error: no member named 'f' in 'B<T>'
    }

    A* operator->();
};
```

This happens because `Sema::ActOnStartCXXMemberReference` does not adjust the `ObjectType` parameter when `ObjectType` is a dependent type (except when the type is a `PointerType` and the class member access is the `->` form). Since the (possibly adjusted) `ObjectType` parameter (`B<T>` in the above example) is passed to `Parser::ParseOptionalCXXScopeSpecifier`, we end up looking up `f` in `B` rather than `A`. 

This patch fixes the issue by identifying cases where the type of the object expression `T` is a dependent, non-pointer type and:
- `T` is the current instantiation and lookup for `operator->` finds a member of the current instantiation, or
- `T` has at least one dependent base case, and `operator->` is not found in the current instantiation

and using `ASTContext::DependentTy` as the type of the object expression when the optional _nested-name-specifier_ is parsed.

Fixes #104268.