Add attributes to document COM interface inheritance

[rickbrew]

We've talked about this on Discord, but having an issue will make this easier to track.

COM interfaces have inheritance relationships, e.g. ID2D1SolidColorBrush inherits from ID2D1Brush, then ID2D1Resource, then IUnknown. In native code you can pass an ID2D1SolidColorBrush* to a method that takes an ID2D1Brush* and it works fine. With C# / TerraFX, you have to cast the pointer every time you do this. The cast is safe, but the compiler can't verify that, and you can easily do the wrong thing such as casting ID2D1Bitmap* to ID2D1Brush*. It'll crash spectacularly at runtime, but nothing at compile-time.

Making this work in TerraFX would be an uphill battle since C# doesn't have any way of making this work, other than maybe auto-generating e.g. struct PIUnknown { IUnknown *p; } wrappers for every interface along with the proper cast operators. And then all your code has to use these in place of pointers, but also needs to use the real pointers at ABI boundaries.

TerraFX doesn't need to include these wrappers and the casting operators, but I could generate them on my own for my own project. However, I can't do that without the inheritance information, which doesn't exist.

So, something like this would be nice in TerraFX:

[NativeInheritance(typeof(ID2D1Brush))]
public struct ID2D1SolidColorBrush
{
    ...
}

Then I can experiment with code-genning my own solutions to this. Anything easily accessible via standard reflection should work.

Some interfaces have multiple interfaces they derive from. Those can be handled by using additional [NativeInheritance(...)]'s.

[NativeInheritance(typeof(IBaseInterface1))]
[NativeInheritance(typeof(IBaseInterface2))]
public struct IMultipleInheritance
    // : IBaseInterface1, IBaseInterface2
{
    ...
}

I'd prefer typeof over strings, e.g. [NativeInheritance("ID2D1Brush")]

[tannergooding]

This should be mostly covered by #193. It is currently using a string as I've not updated ClangSharp to support emitting typeof yet.