[Flang] Upstream conversion of the XRebox Op

The XRebox Op is formed by the codegen rewrite which makes it easier to
convert the operation to LLVM. The XRebox op includes the information
from the rebox op and the associated slice, shift, and shape ops.

During the conversion process a new descriptor is created for reboxing.

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

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D114709