NativeSymbolAttributes inside interfaces inherited by a mixed mode class don't work.

[morguldir]

Describe the bug
NativeSymbol only works inside classes when using a mixed mode class, not inside interfaces

To Reproduce
Steps to reproduce the behavior:

1. Create an interface implementing a function with a different name than the entrypoint.
2. Add a NativeSymbol attribute containing the actual entrypoint.
3. Create an abstract class inheriting from the interface and create an abstract equivalant function.
4. Activate the class.

See https://github.com/Firwood-Software/AdvanceDLSupport/blob/master/docs/mixed_mode_classes.md

Expected behavior
The entrypoint is the same as the one in NativeSymbolAttribute inside interfaces.

Desktop (please complete the following information):

• OS: Linux
• Runtime: .NET Core
• Version: 2.2.0