Made some additional methods of ArcanistLinter and ArcanistExternalLinter final

Summary: To me, it seems that these methods should never be overwritten in subclasses.

Test Plan: arc unit

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D7958