Made some additional methods of ArcanistLintEngine final
Summary: To me, it seems that these methods should not be overridden by subclasses
Test Plan: arc lint and arc unit
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: LegNeato, aran, epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D7960