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

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.

Start by locating the implementation of the [ReinteropNativeImplementation] attribute and the C++/C# interoperation it generates or coordinates. Determine how private fields, properties, and methods are represented across the boundary, then verify that a C++ implementation can access them without changing the C# members to internal.

Written by the indexing model from the issue text.