[flang] Lower alternate return

This patch adds the lowering infrastructure for the lowering of
alternat returns.

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

Depends on D121698

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121699



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