Note [Comparing visibility]

We are sure that we want to distinguish (forall a. ty) and (forall a -> ty); see
Note [ForAllTy and type equality].  But we have /three/ settings for the ForAllTyFlag:
  * Specified: forall a. ty
  * Inferred:  forall {a}. ty
  * Required:  forall a -> ty

We could (and perhaps should) distinguish all three. But for now we distinguish
Required from Specified/Inferred, and ignore the distinction between Specified
and Inferred.

The answer doesn't matter too much, provided we are consistent. And we are consistent
because we always compare ForAllTyFlags with
  * `eqForAllVis`
  * `cmpForAllVis`.
(You can only really check this by inspecting all pattern matches on ForAllTyFlags.)
So if we change the decision, we just need to change those functions.

Why don't we distinguish all three? Should GHC type-check the following program
(adapted from #15740)?

  {-# LANGUAGE PolyKinds, ... #