[libclang] Only mark CXCursors for explicit attributes with a type

All attributes have a source range associated with it. However, implicit
attributes are added by the compiler, and not added because the user
wrote something in the input. So no token type should be set to
CXCursor_*Attr.

The problem was visible when a class gets marked by e.g.
MSInheritanceAttr, which has the full CXXRecordDecl's range as its
own range. The effect of marking that range as CXCursor_UnexposedAttr
was that all cursors for the record decl, including all child decls,
would become CXCursor_UnexposedAttr.

llvm-svn: 330692