[flang] Lower simple character return

Handles function with character return.

Character scalar results are passed as arguments in lowering so
that an assumed length character function callee can access the result
length.

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

Reviewed By: PeteSteinfeld, schweitz

Differential Revision: https://reviews.llvm.org/D120558



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