[flang] Deal with NULL() passed as actual arg to unlimited polymorphic dummy

NULL() passed as actual argument to a procedure with an optional
dummy argument is represented with `fir.box<none>` type. When the dummy
argument is polymoprhic or unlimited polymorphic, the SelectOp will complain
if the types of the two arguments are not identical. Add a conversion from
`fir.box<none>` to `fir.class<none>` in that case.
Other situations with optional will require a fir.rebox and will be done in
a follow up patch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D142203