Resolve super type name in outer context

[soutaro]

Fixes #764 for better compatibility with Ruby.

Note that other type names --self types and generics upper bounds-- continue to be resolved in inner context. This is intentional that we often write types like the following:

module Foo : _Fooable
  interface _Fooable
    def foo: () -> void
  end
end

The _Fooable interface is only used by the Foo module, and we may want to keep it in the module.