[fir] Add character conversion pass

Upstream the character conversion pass.
Translates entities of one CHARACTER KIND to another.
By default the translation is to naively zero-extend or truncate a code
point to fit the destination size.

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

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Valentin Clement <clementval@gmail.com>

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D111405