Detect forward declarations and generate opaque enums for them.

[emilio]

We should be able to detect forward declarations in bindgen, and generate opaque enums if appropriate, instead of structs with an address byte.

We should check clang_getCursorDefinition() and clang_isCursorDefinition to check whether what we're scanning, mark the generated type accordingly, and do the proper codegen steps.

[fitzgen]

#198 is a dupe of this issue, but for posterity it does include an example with a bit more detail than we have in this issue.

[nikhilshagri]

I'll try this one.

[emilio]

Great, let us know if you need anything :)