CppTools: Fix completion for nested enums

Task-number: QTCREATORBUG-5456

Change-Id: I0bb4756e3cdf3c87a4c2b0fbfe6953faaa5e1c52
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>