Fix bison warnings
Summary: Fix a bunch of bison warnings without affecting XHPAST functionality. Depends on D13970.
Test Plan: Compared the MD5 hash of support/xhpast/xhpast before and after this change.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D13974