Fix parsing of the ::class constant

Summary: This updates the grammer to recognize the '::class' constant as valid php. This was added in php 5.5 and results in the full qualified class name.

Reviewers: mwolff, KDevelop

Subscribers: kdevelop-devel, mwolff

Differential Revision: https://phabricator.kde.org/D988

(cherry picked from commit ff5a7d86a3f13634d000bf9efde3d1d4ef8e9326)