Add support in XHPAST for "(new X)->y" syntax (added in 5.4)

Summary: Ref T4334. Allows XHPAST to parse the access-on-instanciation grammar added in 5.4 Refactors some of the other grammar to be closer to Zend's parser. Adds positive and negative tests for allowed syntax.

Test Plan:
`arc unit`
Ran `generate_php_symbols.php` script on files that previously errored out with "unexpected T_OBJECT_OPERATOR", now there are no errors

Reviewers: epriestley, btrahan, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D8150