Note [Canonicity for incoherent matches]

When the selected instance is INCOHERENT at step (IL4) of
Note [Rules for instance lookup], we ignore all unifiers,
whether or not they are marked with INCOHERENT pragmas.
This is implemented by returning NoUnifiers in final_unifs.
NoUnifiers takes an argument indicating whether the match was canonical
as described in Note [Coherence and specialisation: overview] and
Note [Recording coherence information in `PotentialUnifiers`].

To determine whether an incoherent match was canonical, we look *only*
at the OverlapFlag of the instance being matched. For example:

  class C a
  instance {-# INCOHERENT #