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