scheck: do not warn on use of $this in closure, even when not passed in use()

Summary: Paul extended HPHP to not require people to pass $this.

Test Plan:
unit test included
./scheck -test

Reviewers: ptarjan, pieter, amenghra, julienv, zoel

Reviewed By: ptarjan

Differential Revision: https://phabricator.fb.com/D756617