Fix crash on qdbus when return type is a raw dbus type

Assigning a -1 to type is going to make things crash since
it basically means unresolved and when you try to access the string data
go to a index that doesn't exist

So what I do is save the return type in rawReturnType if it is a raw
one and reassign the type to IsUnresolvedType | strings.enter(mm.rawReturnType)
instead of -1 when "saving" the metaobject

Task-number: QTBUG-32671

Change-Id: I67898dea8a1926eee80c16417e877ef4e22aa06b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>