Use unicode arrows with -fprint-unicode-syntax

Summary:
See #8959, this is one more place where we
can pretty-print Unicode syntax.

Test Plan: validate

Reviewers: nomeata, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #8959

Differential Revision: https://phabricator.haskell.org/D5439

compiler/hsSyn/HsTypes.hs

@@ -1444,7 +1444,7 @@ ppr_fun_ty ty1 ty2
   = let p1 = ppr_mono_lty ty1
         p2 = ppr_mono_lty ty2
     in
-    sep [p1, text "->" <+> p2]
+    sep [p1, arrow <+> p2]
 
 --------------------------
 ppr_tylit :: HsTyLit -> SDoc

testsuite/tests/ghci/scripts/T8959b.stderr

@@ -12,5 +12,5 @@ T8959b.hs:8:7: error:
 T8959b.hs:10:7: error:
     • Couldn't match expected type ‘(∀ a2. a2 → a2) → a1’
                   with actual type ‘()’
-    • In the expression: () ∷ (∀ a. a -> a) -> a
-      In an equation for ‘baz’: baz = () ∷ (∀ a. a -> a) -> a
+    • In the expression: () ∷ (∀ a. a → a) → a
+      In an equation for ‘baz’: baz = () ∷ (∀ a. a → a) → a