Fix improper XHPAST parsing of namespace grammar like "use x as private;"
Normally, "grammar_rule" rules emit a standalone Node. In this case, the bottom-level grammar rule is a collection of trivial rules and the callers configure the Node. Wrap the bottom-level rule in a configuration rule so the node is configured correctly and consistently, in exactly one place.
Test Plan: Ran unit tests.
Maniphest Tasks: T13492
Differential Revision: https://secure.phabricator.com/D21068