Allow PHPAST to evalStatic() an n_CONCATENATION_LIST of static nodes

Summary: Ref T1139. Allows evaluation of `pht("a"."b")` -> `ab`.

Test Plan: Successfully evaluated concatenation lists in D8138.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D8139