This fixes Trac #7888, where the user wanted to use 'undefined' in a
context that needed ((forall a. a->a) -> Int).  We allow OpenKind
unification variables to be instantiate with polytypes (or unboxed
types), hence the change.

'error' has always been like this; this change simply extends
the special treatment to 'undefined'.  It's still magical;
you can't define your own wrapper for 'error' and get the
same behaviour.  Really just a convenience hack.