Backpack is unable to type check signatures that expect a data which
is a type level literal. This was reported in issue #15138. These
commits are a fix for this. It also includes a minimal test case that
was mentioned in the issue.

Reviewers: bgamari, ezyang, goldfire

Reviewed By: bgamari, ezyang

Subscribers: simonpj, ezyang, rwbarton, thomie, carter

GHC Trac Issues: #15138

Differential Revision: https://phabricator.haskell.org/D4951