fixes for Win2D to build

[stevenbrix]

These fixes were required to get the Microsoft.Graphics.Canvas WinMD to build, the few things that this winmd uncovered:

1. Simply deriving from an interface resulted in not properly tracking dependencies.
2. subscript is a method and needs to be escaped with backticks
3. There are static methods which technically "override" others (they have the same name, num of params and return types are related). These need to be marked as class methods and the derived types have to say override

Changes

1. add w.add_depends in some places where we don't use any of the write(type) methods. We don't use those methods because swift name requirements for deriving from a generic interface don't follow "normal" (as in what most other languages do - where you need IFoo instead of IFoo<Bar>) naming patterns. Plus those write methods are fragile so just adding the add_depends instead of modifying the write methods to do the right thing
2. add subscript to the list of names that need backticks
3. this one is a bit more involved, we have to walk the class hierarchy and see if a function matches. This is similar to what we do for base_has_matching_constructor and a match is considered if the following are true:
   a. function name matches
   b. parameters match
   c. return type of derived static method is the same or derived from the return of the base class

Testing

Added test cases

[darinf]

LGTM!