[flang] Add conditional rebox when passing fir.box to optional fir.class

When a `!fir.box<>` is passed as an actual argument to an optional
`!fir.class<>` dummy it needs a `fir.rebox` in order to propagate
the dynamic type information.
The `fir.rebox` needs to happen only on present argument.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D142340