Debugger: Correctly identify derived classes from references

Previously the correctly gdb-identified dynamic types were overwritten
with the value's static type. This is probably because the dynamic type
doesn't include the "&" for "reference". That, however, can easily be
fixed by just appending "&". As we're only handling references there it
should be safe to do so.

Task-number: QTCREATORBUG-10888
Change-Id: I7310916ce662956e66491423ad26658c32c8776b
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>