Fix dependencies among specialisations for imported Ids

This was a subtle one (Trac #4903).  See
  Note [Glom the bindings if imported functions are specialised]
in Speclialise.

Fundamentally, a specialised binding for an imported Id was being
declared non-recursive, whereas in fact it can become recursive
via a RULE.  Once it's specified non-recurive the OccAnal pass
treats that as gospel -- and that in turn led to infinite inlining.

Easily fixed by glomming all the specialised bindings in a Rec;
now the OccAnal will sort them out correctly.