[flang] Lower length on character storage

This patch adds lowering for SetLength used to set
different length on character storage around calls where
the dummy and actual length differ.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D122000



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