Item_direct_view_ref::derived_field_transformer_for_where
           upon updating a view

The condition pushed into a materialized derived / view mast be adjusted
for the new context: its column references must be substituted for
references to the columns of the underlying tables if the condition
is pushed into WHERE. The substitution is performed by the 'transform'
method. If the materialized derived is used in a mergeable view then
the references to the columns of the view are represented by
Item_direct_view_ref objects. The transform method first processes
the item wrapped in such an object and only after this it transforms
the object itself.
The transformation procedure of an Item_direct_view_ref object has
to know whether the item it wraps has been substituted. If so the
procedure does not have to do anything. In the code before this patch
it was not possible for the transformation procedure used by an
Item_direct_view_ref object to find out whether a substitution for
the wrapped item had happened.