[fir] Add Character helper

This patch is extracted from D111337. It introduce the
CharacterExprHelper that helps dealing with character in FIR.

Reviewed By: schweitz, awarzynski

Differential Revision: https://reviews.llvm.org/D112140



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