[IR] Create a toplevel IRScope and bind it to the toplevel staticscope

* TOPLEVEL_BINDING was trying to fetch the IRScope associated with
  it and was coming back empty and crashing bundler in some scenarios.

  Curious why it took this particular combination to uncover this
  longstanding initialization bug.

* Longer term 9k cleanup TODO:
  - Merge IRStaticScope and IRScope -- there are two of them because
    of having to co-exist with the earlier runtime. We don't have
    that constraint anymore.