Note [HasGivenEqs]

The GivenEqs data type describes the Given constraints of an implication constraint:

* NoGivenEqs: definitely no Given equalities, except perhaps let-bound skolems
  which don't count: see Note [Let-bound skolems] in GHC.Tc.Solver.InertSet
  Examples: forall a. Eq a => ...
            forall a. (Show a, Num a) => ...
            forall a. a ~ Either Int Bool => ...  -- Let-bound skolem

* LocalGivenEqs: definitely no Given equalities that would affect principal
  types.  But may have equalities that affect only skolems of this implication
  (and hence do not affect principal types)
  Examples: forall a. F a ~ Int => ...
            forall a b. F a ~ G b => ...

* MaybeGivenEqs: may have Given equalities that would affect principal
  types
  Examples: forall. (a ~ b) => ...
            forall a. F a ~ b => ...
            forall a. c a => ...       -- The 'c' might be instantiated to (b ~)
            forall a. C a b => ....
               where class x~y => C a b
               so there is an equality in the superclass of a Given

The HasGivenEqs classifications affect two things:

* Suppressing redundant givens during error reporting; see GHC.Tc.Errors
  Note [Suppress redundant givens during error reporting]

* Floating in approximateWC.

Specifically, here's how it goes:

                 Stops floating    |   Suppresses Givens in errors
                 in approximateWC  |
                 
 NoGivenEqs         NO             |         YES
 LocalGivenEqs      NO             |         NO
 MaybeGivenEqs      YES            |         NO