[flang] Lower allocatable assignment for scalar

Add lowering for simple assignement on allocatable
scalars.

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

Depends on D120483

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D120488



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