It is not sufficient that QSharedPointer<ForwardDeclared>'s default ctor
and dtor compile. Copy/move assignment/construction and swapping should work, too.
Arguably, there are more functions that should compile with just a forward-declared
payload, but this is a start.
Change-Id: I75470e3d4ba949c3e735c4078cbc123d53ec3007
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>