[flang] Lower more array character cases

This patch adds more lowering and tests for character array assignment/copy.

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

Depends on D121300

Reviewed By: PeteSteinfeld, schweitz

Differential Revision: https://reviews.llvm.org/D121301



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