Minor change to ArcanistXHPASTLinter

Summary: Change == to ===. This is a little bit cleaner because we shouldn't need type coercion here.

Test Plan: Ran arc unit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9893