Thrust 1.8.3 uses raw_reference_cast instead of a direct assignment operator

This usage of raw_referenc_cast returns a const PortalValue which is than
assigned too. So to work around the problem we need to mark operator= on
the class as const.

vtkm/exec/cuda/internal/IteratorFromArrayPortal.h

@@ -65,7 +65,7 @@ struct PortalValue
   }
 
   VTKM_EXEC_EXPORT
-  ValueType operator=(const ValueType& value)
+  ValueType operator=(const ValueType& value) const
   {
     this->Portal.Set(this->Index, value);
     return value;