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