Summary:
GHC was needlessly rejecting GADT constructors' type
signatures that were surrounded in parentheses due to the fact that
`splitLHsForAllTy` and `splitLHsQualTy` (which are used to check as
part of checking if GADT constructor return types are correct)
weren't looking through parentheses (i.e., `HsParTy`). This is
easily fixed, though.

Test Plan: make test TEST=T14320

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #14320

Differential Revision: https://phabricator.haskell.org/D4072