Mark some methods in `ArcanistLinter` as final

Summary: Minor change, self-explanatory... this seems to make sense.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10476