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