Rename the types in a GADT constructor in toposorted order

Previously, we were extracting the free variables from a
GADT constructor in an incorrect order, which caused the type
variables for the constructor's type signature to end up in
non-toposorted order. Thankfully, rearranging the order of types
during renaming makes swift work of this bug.

This fixes a regression introduced in commit
fa29df02.
For whatever reason, that commit also commented out a
significant portion of the `T13123` test. This code appears
to work, so I've opted to uncomment it.

Test Plan: make test TEST=T14808

Reviewers: simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14808

Differential Revision: https://phabricator.haskell.org/D4413