This adds the tyvars of the domain of the substitution into the in-scope
set as well.
What I'm not sure here is if the kinds can have any free vars that
should be in the in-scope set as well.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D2094

GHC Trac Issues: #11371