[flang] Lower entry statement

This patch add the lowering for the entry statement.

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

Depends on D121697

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121698



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