[flang] Add type conversion for !fir.box<none>

`none` is used in `fir.box` type to specify a polymorphic type.
This patch add the conversion from `!fir.box<none>` to LLVM.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D119325



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>