Warn when dynamically subclassing a static class

[vihanb]

Certain classes are 'static' for design purposes. This means that with this class:

@dynamic(false)
public class Animal {
    // ...

    func makeSound() { ... }
}

The following should warn:

public class Dog: Animal {
    override public func makeSound() { ... }
}

Warning: Subclass of a static class cannot be dynamic, add `@dynamic(false)` (1:0)

    > 1 | public class Dog: Animal {
        | ^
      2 |     override public func makeSound() { ... }
      3 | }