Reinterop: Allow C++ implementation classes to access private fields / properties / methods of C# class

[kring]

When using the [ReinteropNativeImplementation] attribute, the C++ and C# classes are conceptually one class, just with parts implemented in two different languages. To make that more seamless, it would be nice if the C++ code could access private members of the C# class. Currently the only way to do this is by declaring the private members "internal" instead.