[flang] Fix forall issue with substring operation

When there is a substring operation on a scalar assignment in a FORALL
context, we have to lower the entire substring and not the entire
CHARACTER variable.

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

Reviewed By: PeteSteinfeld, klausler

Differential Revision: https://reviews.llvm.org/D128459



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