[flang] Add TargetRewrite pass

This patch adds the basic infrastructure for the TargetRewrite pass,
which rewrites certain FIR dialect operations into target specific
forms. In particular, it converts boxchar function parameters, call
arguments and return values. Other convertions will be included in
future patches.

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

Differential Revision: https://reviews.llvm.org/D112910



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>
Co-authored-by: Tim Keith <tkeith@nvidia.com>