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