Resolve overrides for Obj-C descriptors
Minor refactoring in Obj-C resolve

Use MethodKind enum instead of manually copying methods to two lists (class
methods and instance methods)
Resolve Obj-C metaclasses into separate classes

Since Objective-C metaclasses inherit from each other and with our class
objects it doesn't seem to be possible, for every Objective-C class A we now
create three descriptors: open class A, abstract trait <metaclass-for-A> and
final class object <class-object-for-A>, which inherits from <metaclass-for-A>
Don't create empty class object for Obj-C metaclass