Allow things to figure out if --trace was passed

Summary:
I want to make bin/phd debug only activate tracing if it was called with --trace, but there's no easy way to ask "is tracing enabled?" right now.

Provide such a method.

Test Plan: See next revision.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15422