In MSVC compatibility mode, friend function declarations behave as function declarations

Before C++20, MSVC treated any friend function declaration as a function declaration, so the following code would compile despite funGlob being declared after its first call:

```
class Glob {
public:
  friend void funGlob();

  void test() {
    funGlob();
  }
};

void funGlob() {}
```
This proposed patch mimics the MSVC behavior when in MSVC compatibility mode

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D124613