In MSVC compatibility mode, handle unqualified templated base class initialization

Before C++20, MSVC was supporting not mentioning the template argument of the base class when initializing a class inheriting a templated base class.
So the following code compiled correctly:
```
template <class T>
class Base {
};

template <class T>
class Derived : public Base<T> {
public:
  Derived() : Base() {}
};

void test() {
    Derived<int> d;
}
```
See https://godbolt.org/z/Pxxe7nccx for a conformance view.

This patch adds support for such construct when in MSVC compatibility mode.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D124666