Fix moc generating invalid code for slots with reference types as argument.

Fix moc generating invalid code for slots with reference types as argument.

We can't have T& declared/registered as a metatype (wont compile), but
using it as type for a slot argument is possible. With the recent
introduction of metatype auto-registration we have to make sure that moc
doesn't attempt to auto-register those. Simple types are handled correctly
already, this fixes containers and smart pointers.

Change-Id: Id96857c57d6ebf158a67e9d527c89dc195473b1b
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>