CXCursor value for __interface

I'm reworking the __interface support patch as a keyword (rather than
attribute off of struct) and see I need to give it a CXCursor value,
e.g. in clang::getCursorKindForDecl.

I see all the CXCursor values are explicitly set and are part of a
public interface, so it doesn't seem like I can bump them to insert a
value for interface - can I? CXCursor_LastDecl is immediately before
CXCursor_FirstRef, so I'd have to change one to fit in a new Decl value.
For now I'm using CXCursor_StructDecl; is it fine to stick with that?
Should I use CXCursor_UnexposedDecl?

Definitely not unexposed. If we're really discriminating by keyword here,
then StructDecl is fine.