Make the methods of `ArcanistBaseXHPASTLinter` final.

Summary: No subclass should need to override these methods. Additionally, none currently do.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9035