Don't include debugging symbols in XHPAST by default

Summary: I'm not C/C++ expert, but it seems odd to me that we include debugging symbols on a normal xhpast build... doesn't this have some performance overhead?

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13596