Use reference to force value conversion.
Use reference to force value conversion.

Previously, the transform_iterator implementation would make a copy
of the adapted iterators value when dereferencing it in order to
force conversion to the value_type. This prevented transform_iterators
over non-copyable types. Using a reference instead allows for forcing
the conversion without invoking a copy ctor.

thrust/thrust,thrust/thrust,thrust/thrust,thrust/thrust,thrust/thrust