[flang] Fix bug in character casting. Add missing sext/trunc in code gen.

This patch is part of the upstreaming effort from fir-dev branch.
It also ensures all descriptors created inline complies with LBOUND
requirement that the lower bound is `1` when the related dimension
extent is zero.

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

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128047



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