[fir] Add fir.box type conversion

This patch add the `!fir.box` type conversion to llvm.
`fir.box` is converted to the descriptor as defined in the ISO_Fortran_binding.h
and the addendum defined in descriptor.h.

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

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D113288



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