It doesn't look like we keep any sets of free variables
of the types of Ids handy, so we just have to build them
when doing a substitution.

Test Plan: buildbot + run testsuite with debug

Reviewers: simonmar, simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: carter, rwbarton, thomie

GHC Trac Issues: #11371

Differential Revision: https://phabricator.haskell.org/D3431