SI-8803 generate super accessor for super[A] where A is superclass

class C extends A with T {
  class I {
    super[T]
    super[A]
  }
}

A super call in a nested class of the form super[T] where T is a
parent trait of the outer class doesn't need an accessor: mixin can
directly re-route the call to the correct implementation class - it's
statically known to be T$class.

However, if a nested class accesses super[A] and A is the superclass
of the outer class (not a trait), then we need a super accessor in the
outer class.

We need to add the mixin name to the super accessor name, otherwise
it clashes with non-qualified super accessors.