Write a linter rule for $this reassignment
Summary: Re-assigning $this is an invalid PHP construct, see https://3v4l.org/TauX9.
Test Plan: Added unit tests.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14514