This amounts to a one-line change in `tcExpr`. I've added a Note to
explain what is going on.

This requires a separate change in the pattern-match checker to
account for the fact that typechecked `[]` expressions become
`ConLikeOut`s, not `ExplicitList`s.

Test Plan: make test TEST=T13680

Reviewers: goldfire, mpickering, austin, bgamari

Reviewed By: mpickering, bgamari

Subscribers: rwbarton, thomie, goldfire

GHC Trac Issues: #13680

Differential Revision: https://phabricator.haskell.org/D3733