cast_value: assign the new type

The first two arguments of cast_value() are the new expression and the
type wanted for it. This type is then used to calculate the new value.

But the type of the expression must be assigned separately (usually
after the cast because the old and the new expression can refer to
the same object).

To avoid any possible inconsistencies, assign the new type during the
casting itself.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>