Correctly support try..catch..finally in xhpast
Summary: Ref T4334. The catch clauses are optional when a finally is available.
Test Plan: run xhpast with some files. Also added unit tests
Reviewers: joshuaspence, epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: joshuaspence, epriestley, Korvin
Maniphest Tasks: T4334
Differential Revision: https://secure.phabricator.com/D10337