Need check that overload name is not used in subtype

[mikesamuel]

interface I {
  @overload("f")
  let g(): Void;
}

class C extends I {
  public g(): Void {}
  public f(): Void {}  // Illegal
}

The presence of @overload for a name in a super-type basically reserves that name in sub-types. This is meant to allow backends to do extra things with the overload name without also managing name conflicts.

[tjpalmer]

There's also this edge case, which should be an error:

interface I {
  let f(): Void;
}

class C extends I {
  @overload("f")
  public g() Void {}

  @overload("f")
  public h(): Void {}
}

Someone could make an adapter type extending I instead, for example.