Micro-optimization for linting performance

Summary: This is a micro-optimization to improve the performance of the XHPAST linter.

Test Plan: Ran arc lint --everything --never-apply-patches --xprofile=profile.json in rP. Compared the XHProf profile before and after.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13876