Syntax support for grouped namespaces

This is a first step to implement Grouped namespaces.

The syntax has been updated to parse Grouped namespaces without generating errors,
but declarations into grouped namespaces are not processed.

"Normal" use statements are still processed.

CCBUG: 411588
CCBUG: 459654
CHANGELOG: Add partial support for PHP 7.0's grouped namespace syntax

Reviewers: pprkut

Reviewed By: pprkut

Subscribers: pprkut, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D29444