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