Further streamlining of interpreter context

* Deleted old code in IRScope that is no longer relevant.

* For closures, short-circuited prepareForInterpretation from
  base class to ensure that callers use the 1-arg version where
  it needs to be built.

* Subsequent crashers exposed an unresolved FIXME in BuildLambdaInstr
  that had a WrappedIRClosure that needed cloning and build of an
  interpreter context as well.