Make the Con and Con' patterns produce evidence

Matching with the `Con` and `Con'` patterns can reveal evidence
that the type in question is *not* an application. This can help
the pattern checker.

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: carter, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4139