Refactor hole constraints.

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, 15 insertions, 1 deletions

diff --git a/compiler/GHC/Tc/Solver/Flatten.hs b/compiler/GHC/Tc/Solver/Flatten.hs
index 551e1de395..4dff585840 100644
--- a/compiler/GHC/Tc/Solver/Flatten.hs
+++ b/compiler/GHC/Tc/Solver/Flatten.hs
@@ -5,7 +5,7 @@
 module GHC.Tc.Solver.Flatten(
    FlattenMode(..),
    flatten, flattenKind, flattenArgsNom,
-   rewriteTyVar,
+   rewriteTyVar, flattenType,
 
    unflattenWanteds
  ) where
@@ -825,6 +825,20 @@ flattenArgsNom ev tc tys
        ; traceTcS "flatten }" (vcat (map ppr tys'))
        ; return (tys', cos, kind_co) }
 
+-- | Flatten a type w.r.t. nominal equality. This is useful to rewrite
+-- a type w.r.t. any givens. It does not do type-family reduction. This
+-- will never emit new constraints. Call this when the inert set contains
+-- only givens.
+flattenType :: CtLoc -> TcType -> TcS TcType
+flattenType loc ty
+          -- More info about FM_SubstOnly in Note [Holes] in GHC.Tc.Types.Constraint
+  = do { (xi, _) <- runFlatten FM_SubstOnly loc Given NomEq $
+                    flatten_one ty
+                     -- use Given flavor so that it is rewritten
+                     -- only w.r.t. Givens, never Wanteds/Deriveds
+                     -- (Shouldn't matter, if only Givens are present
+                     -- anyway)
+       ; return xi }
 
 {- *********************************************************************
 *                                                                      *