Issue Description:

The following code snippet:

public class C { }
extension C {
  open func f() { }
}

compiles without a diagnostic today. Superficially, it might seem like you can override f from a subclass of C. However, that's not the case; you can only overload it. Being able to override methods declared in extensions would require a mechanism like objc_msgSend, which is why such overloading only works when @objc is present, there is no Swift equivalent for pure Swift classes.

This means that the open is misleading to a reader; we should issue a diagnostic with a fix-it to replace this with public instead in such situations.