Add assistant to generate header guards
Summary:
Assistant with two actions allowing you to either add a conventional macro-
based header guard or pragma-once.
Test Plan: Run test_assistants testHeaderGuardAssistant
Reviewers: KDevelop, arrowd
Reviewed By: arrowd
Subscribers: apol, arrowd, kdevelop-devel
Tags: KDevelop
Differential Revision: https://phabricator.kde.org/D17370