Don't put foralls in front of TH-spliced GADT constructors that don't need them

Summary:
It turns out that D2974 broke this program
(see https://phabricator.haskell.org/rGHC729a5e452db5#58801):

```lang=haskell
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

import GHC.Exts (Constraint)

$([d| data Dec13 :: (* -> Constraint) -> * where
        MkDec13 :: c a => a -> Dec13 c
    |])
```

This was actually due to a long-standing bug in `hsSyn/Convert` that put
unnecessary `forall`s in front of GADT constructors that didn't have any
explicitly quantified type variables.

This cargo-cults the code in `Convert` that handles `ForallT` and adapts
it to `ForallC`. Fixes #13123 (for real this time).

Test Plan: make test TEST=T13123

Reviewers: goldfire, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3002

GHC Trac Issues: #13123

1 files changed, 7 insertions, 0 deletions

diff --git a/testsuite/tests/th/T13123.hs b/testsuite/tests/th/T13123.hs
index 987283be70..d7e1006b9e 100644
--- a/testsuite/tests/th/T13123.hs
+++ b/testsuite/tests/th/T13123.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
@@ -5,6 +6,8 @@
 {-# LANGUAGE TemplateHaskell #-}
 module T13123 where
 
+import GHC.Exts (Constraint)
+
 $([d| idProxy :: forall proxy (a :: k). proxy a -> proxy a
       idProxy x = x
     |])
@@ -28,3 +31,7 @@ $([d| class Foo b where
 $([d| data GADT where
         MkGADT :: forall proxy (a :: k). proxy a -> GADT
     |])
+
+$([d| data Dec13 :: (* -> Constraint) -> * where
+        MkDec13 :: c a => a -> Dec13 c
+    |])