Fixes the warning about incompatible pointer types on assigning to a
subclass of type argument an expression of type __kindof TypeParam.
We already have a mechanism in ASTContext::canAssignObjCInterfaces
that handles ObjCObjectType with __kindof. But it wasn't triggered
because during type substitution __kindof TypeParam was represented as
AttributedType with attribute ObjCKindOf and equivalent type
TypeArg. For assignment type checking we use canonical types so
attributed type was desugared and the attribute was ignored.
The fix is in checking transformed AttributedType and pushing
__kindof down into ObjCObjectType when necessary.