Note [Equality superclasses in quantified constraints]

Consider (#15359, #15593, #15625)
  f :: (forall a. theta => a ~ b) => stuff

It's a bit odd to have a local, quantified constraint for `(a~b)`,
but some people want such a thing (see the tickets). And for
Coercible it is definitely useful
  f :: forall m. (forall p q. Coercible p q => Coercible (m p) (m q)))
                 => stuff

Moreover it's not hard to arrange; we just need to look up /equality/
constraints in the quantified-constraint environment, which we do in
GHC.Tc.Solver.Equality.tryQCsEqCt.

There is a wrinkle though, in the case where 'theta' is empty, so
we have
  f :: (forall a. a~b) => stuff

Now, potentially, the superclass machinery kicks in, in
makeSuperClasses, giving us a a second quantified constraint
       (forall a. a ~# b)
BUT this is an unboxed value!  And nothing has prepared us for
dictionary "functions" that are unboxed.  Actually it does just
about work, but the simplifier ends up with stuff like
   case (/\a. eq_sel d) of df -> ...(df @Int)...
and fails to simplify that any further.

So for now we simply decline to take superclasses in the quantified
case.  Instead we have a special case in GHC.Tc.Solver.Equality.tryQCsEqCt
which looks for primitive equalities specially in the quantified
constraints.

See also Note [Evidence for quantified constraints] in GHC.Core.Predicate.