[flang] Lower more cases of assignments on allocatable variables

This patch enables the lowering of various allocatable assignements
for character type and numeric types.

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

Depends on D120819

Reviewed By: PeteSteinfeld, schweitz

Differential Revision: https://reviews.llvm.org/D120820



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