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