Note [Instance and Given overlap]

Example, from the OutsideIn(X) paper:
       instance P x => Q [x]
       instance (x ~ y) => R y [x]

       wob :: forall a b. (Q [b], R b a) => a -> Int

       g :: forall a. Q [a] => [a] -> Int
       g x = wob x

From 'g' we get the implication constraint:
            forall a. Q [a] => (Q [beta], R beta [a])
If we react (Q [beta]) with its top-level axiom, we end up with a
(P beta), which we have no way of discharging. On the other hand,
if we react R beta [a] with the top-level we get  (beta ~ a), which
is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is
now solvable by the given Q [a].

The partial solution is that:
  In matchClassInst (and thus in topReact), we return a matching
  instance only when there is no Given in the inerts which is
  unifiable to this particular dictionary.

  We treat any meta-tyvar as "unifiable" for this purpose,
  *including* untouchable ones.  But not skolems like 'a' in
  the implication constraint above.

The end effect is that, much as we do for overlapping instances, we
delay choosing a class instance if there is a possibility of another
instance OR a given to match our constraint later on. This fixes
tickets #4981 and #5002.

Other notes:

* The check is done *first*, so that it also covers classes
  with built-in instance solving, such as
     - constraint tuples
     - natural numbers
     - Typeable

* See also Note [What might equal later?] in GHC.Tc.Utils.Unify

* The given-overlap problem is arguably not easy to appear in practice
  due to our aggressive prioritization of equality solving over other
  constraints, but it is possible. I've added a test case in
  typecheck/should-compile/GivenOverlapping.hs

* Another "live" example is #10195; another is #10177.

* We ignore the overlap problem if -XIncoherentInstances is in force:
  see #6002 for a worked-out example where this makes a
  difference.

* Moreover notice that our goals here are different than the goals of
  the top-level overlapping checks. There we are interested in
  validating the following principle:

      If we inline a function f at a site where the same global
      instance environment is available as the instance environment at
      the definition site of f then we should get the same behaviour.

  But for the Given Overlap check our goal is just related to completeness of
  constraint solving.

* The solution is only a partial one.  Consider the above example with
       g :: forall a. Q [a] => [a] -> Int
       g x = let v = wob x
             in v
  and suppose we have -XNoMonoLocalBinds, so that we attempt to find the most
  general type for 'v'.  When generalising v's type we'll simplify its
  Q [alpha] constraint, but we don't have Q [a] in the 'givens', so we
  will use the instance declaration after all. #11948 was a case
  in point.

All of this is disgustingly delicate, so to discourage people from writing
simplifiable class givens, we warn about signatures that contain them;
see GHC.Tc.Validity Note [Simplifiable given constraints].