Summary:
#11721 changed the order of type variables in GADT
constructor type signatures, but these changes weren't reflected in
Template Haskell reification of GADTs. Let's do that.

Along the way, I:

* noticed that the `T13885` test was claiming to test TH reification
  of GADTs, but the reified data type wasn't actually a GADT! Since
  this patch touches that part of the codebase, I decided to fix
  this.
* incorporated some feedback from @simonpj in
  https://phabricator.haskell.org/D3687#113566. (These changes alone
  don't account for any different in behavior.)

Test Plan: make test TEST=T11721_TH

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: goldfire, bgamari, simonpj

Subscribers: rwbarton, thomie, simonpj

GHC Trac Issues: #11721

Differential Revision: https://phabricator.haskell.org/D4070