We dump it in the interface file, so we need to do it in a
deterministic order. I haven't seen any problems with this
during my testing, but that's probably because it's unused.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2313

GHC Trac Issues: #4012