This is the fix proposed in #11371:
```
In other cases, we already have the in-scope set in hand. Example: in
CoreLint.lintTyApp we find a call to substTyWith. But Lint carries an
in-scope set, so it would be easy to pass it to substTyWith.
```

Test Plan: ./validate --slow (only pre-existing problems)

Reviewers: simonpj, goldfire, austin, nomeata, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1820

GHC Trac Issues: #11371