[fir] Add fir character builder

This patch adds the FIR builder to generate the numeric intrinsic
runtime call.

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

Reviewed By: rovka

Differential Revision: https://reviews.llvm.org/D114900



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