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