[flang] Fix bugs relating to support for characters of different kinds

Fix bugs relating to support for characters of different kinds. Lowering
was creating bad FIR and MLIR that crashed in conversion to LLVM IR.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D128723



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