Support class const dereferencing in xhpast
Ref T4334. As it turns out, a major overhaul of the syntax wasn't needed.
This adds support for stuff like SomeClass::SOME_CONST[$idx] which was introduced in PHP 7.
Test Plan: Added a test case. Ran tests on libphutil and arcanist with both PHP 5 and PHP 7.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, Firehed
Maniphest Tasks: T4334
Differential Revision: https://secure.phabricator.com/D16138