The `substTheta` call didn't have the free variables of the
`prov_theta` in the `in_scope` set. It should be enough to add
`univ_tvs`, as all the `ex_tvs` are already in the domain of
the substitution.

Test Plan: added a testcase

Reviewers: simonpj, bgamari, goldfire, austin

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1879

GHC Trac Issues: #11524