Fix mkGadtDecl does not set con_forall correctly

Authored by alanz on Fri, Jun 29, 1:58 PM.

Description

Fix mkGadtDecl does not set con_forall correctly

A GADT declaration surrounded in parens does not det the con_forall
field correctly.

e.g.

data MaybeDefault v where

TestParens  :: (forall v . (Eq v) => MaybeDefault v)

Closes Trac #15323

(cherry picked from commit 6e4e6d1c674a9d0257ca5c6caa26da18edf8ad8c)