CppTools: Offer "override" and "final" as completion items

These are special identifiers and were thus not included in the 'add all
keywords' handling.

Change-Id: I7f65df5711193e945bfa955dcb70a6ab454606b5
Task-number: QTCREATORBUG-11341
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>