Use cppcheck to lint XHPAST source
Summary: Use Cppcheck to lint XHPAST source files.
Test Plan: Ran arc lint --trace support/xhpast/xhpast.cpp and saw cppcheck invoked.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12272