Fix XHPAST parsing of variadic calls

Summary:
Depends on D21066. Ref T13492. The switch of unit test data to stable/readable output exposed this bug in parsing of variadic calls: some nodes are not given types properly.

Fix the parser and update the test.

Test Plan: Ran the test, which now works.

Maniphest Tasks: T13492

Differential Revision: https://secure.phabricator.com/D21067