C++ Interop: Add test coverage for access control of static/instance data/function members (#6199)

This demonstrates two issues:
1. It seems like we wrongly treat private static data members the same
way we treat protected and allow access to them from within derived
classes member functions.
2. Calling instance member functions of a base C++ class using a derived
class as self (no implicit upcast) is not yet supported. This isn't
related to access control, but prevents us from testing some access
control use cases.

Part of #5859.

Author: bricknerb