Issue Description:

Starting with a disclaimer that I'm absolutely not sure if this is somehow intended behaviour, but I very much think it's not at this point.

The problem is that I'm having a UIView-Subclass in let's call it `UITools`. As it's descending from UIView, no `@objc` is needed and I can use it as the type for an `@IBOutlet` freely in any dependant app or framework target – this works both with and without `-enable-library-evolution`. The problem arises only when I'm subclassing something that has been defined in `UITools` from within a dependant target, example below:

In UITools:

open class CustomButton: UIButton{  
}

In App:

class EvenMoreCustomButton: CustomButton {
   } 
 
class ViewController: UIViewController{     
   @IBOutlet var button: CustomButton? // Compiles with no problems     
   @IBOutlet var button: EvenMoreCustomButton? // IBOutlet property cannot have non-`@objc` class type..  
}

Now, recompiling `UITools` without `-enable-library-evolution` fixes the compilation problem, so it seems to be that compiling frameworks for distribution actually breaks the @objc-inference in dependant projects.

Note: For that specific example, a deployment target below iOS 13 must be used to illustrate the problematic behaviour.