Refactor hole constraints.wip/hole-refactor

Previously, holes (both expression holes / out of scope variables and
partial-type-signature wildcards) were emitted as *constraints* via
the CHoleCan constructor. While this worked fine for error reporting,
there was a fair amount of faff in keeping these constraints in line.
In particular, and unlike other constraints, we could never change
a CHoleCan to become CNonCanonical. In addition:
 * the "predicate" of a CHoleCan constraint was really the type
   of the hole, which is not a predicate at all
 * type-level holes (partial type signature wildcards) carried
   evidence, which was never used
 * tcNormalise (used in the pattern-match checker) had to create
   a hole constraint just to extract it again; it was quite messy

The new approach is to record holes directly in WantedConstraints.
It flows much more nicely now.

Along the way, I did some cleaning up of commentary in
GHC.Tc.Errors.Hole, which I had a hard time understanding.

This was instigated by a future patch that will refactor
the way predicates are handled. The fact that CHoleCan's
"predicate" wasn't really a predicate is incompatible with
that future patch.

No test case, because this is meant to be purely internal.

It turns out that this change improves the performance of
the pattern-match checker, likely because fewer constraints
are sloshing about in tcNormalise. I have not investigated
deeply, but an improvement is not a surprise here:

-------------------------
Metric Decrease:
    PmSeriesG
-------------------------

1 files changed, 6 insertions, 7 deletions

diff --git a/compiler/GHC/Tc/Solver/Interact.hs b/compiler/GHC/Tc/Solver/Interact.hs
index 6a391d4406..d95c13cd54 100644
--- a/compiler/GHC/Tc/Solver/Interact.hs
+++ b/compiler/GHC/Tc/Solver/Interact.hs
@@ -195,7 +195,7 @@ solve_simple_wanteds :: WantedConstraints -> TcS (Int, WantedConstraints)
 -- Try solving these constraints
 -- Affects the unification state (of course) but not the inert set
 -- The result is not necessarily zonked
-solve_simple_wanteds (WC { wc_simple = simples1, wc_impl = implics1 })
+solve_simple_wanteds (WC { wc_simple = simples1, wc_impl = implics1, wc_holes = holes })
   = nestTcS $
     do { solveSimples simples1
        ; (implics2, tv_eqs, fun_eqs, others) <- getUnsolvedInerts
@@ -204,7 +204,8 @@ solve_simple_wanteds (WC { wc_simple = simples1, wc_impl = implics1 })
             -- See Note [Unflatten after solving the simple wanteds]
        ; return ( unif_count
                 , WC { wc_simple = others `andCts` unflattened_eqs
-                     , wc_impl   = implics1 `unionBags` implics2 }) }
+                     , wc_impl   = implics1 `unionBags` implics2
+                     , wc_holes  = holes }) }
 
 {- Note [The solveSimpleWanteds loop]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -264,7 +265,7 @@ runTcPluginsGiven
 -- 'solveSimpleWanteds' should feed the updated wanteds back into the
 -- main solver.
 runTcPluginsWanted :: WantedConstraints -> TcS (Bool, WantedConstraints)
-runTcPluginsWanted wc@(WC { wc_simple = simples1, wc_impl = implics1 })
+runTcPluginsWanted wc@(WC { wc_simple = simples1 })
   | isEmptyBag simples1
   = return (False, wc)
   | otherwise
@@ -285,11 +286,10 @@ runTcPluginsWanted wc@(WC { wc_simple = simples1, wc_impl = implics1 })
 
        ; mapM_ setEv solved_wanted
        ; return ( notNull (pluginNewCts p)
-                , WC { wc_simple = listToBag new_wanted       `andCts`
+                , wc { wc_simple = listToBag new_wanted       `andCts`
                                    listToBag unsolved_wanted  `andCts`
                                    listToBag unsolved_derived `andCts`
-                                   listToBag insols
-                     , wc_impl   = implics1 } ) } }
+                                   listToBag insols } ) } }
   where
     setEv :: (EvTerm,Ct) -> TcS ()
     setEv (ev,ct) = case ctEvidence ct of
@@ -494,7 +494,6 @@ interactWithInertsStage wi
              CIrredCan {} -> interactIrred   ics wi
              CDictCan  {} -> interactDict    ics wi
              _ -> pprPanic "interactWithInerts" (ppr wi) }
-                -- CHoleCan are put straight into inert_frozen, so never get here
                 -- CNonCanonical have been canonicalised
 
 data InteractResult