The pre-condition on `mkTopTCvSubst` turned out to be wrong and
not satisfied by any of the callers. I've fixed it, so that it
constructs the in_scope set from the range of the substitution.
`mkTopTCvSubst` was also unnecessarily general it is never called
with `CoVars`, so I changed the type signature and added an assertion.

Test Plan: ./validate --slow

Reviewers: goldfire, simonpj, bgamari, austin

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1801

GHC Trac Issues: #11371