Add macro to skip including "MyClass.g.h" etc

[roxk]

Idea: The bulk of idl generation is actually clang parsing all header files. If we can skip parsing them, idl generation would be instant.

Pass attempt includes removing .g.h line manually and inserting a bunch of dummy struct ClassT {} classes. I forgot why I didn't explore further but there was some roadblock that necessitates the current bootstrap approach.

Maybe with the help of conditional header inclusion, conditional class content based on macro, the previous dummy ClassT insertion approach would work.