Disallow SuperTypes as prefixes of TypeRefs.

[odersky]

It's not clear what that means anyway. The classical use case

class O {
  class I 
}

class S extends O { class I extends super.I }

is no longer supported, since classes cannot be overridden nor shadowed. This also needs to be checked.

[odersky]

With the new semantics of classes, we do allow same-named inner classes, at least in Scala-2 mode. So it's not clear whether we should support this or not.