Relax text linter rules for XHPAST source code

Summary: This code doesn't comply with the 80 character limit, so either relax or fix?

Test Plan: `arc lint`... saw less errors.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11058