[flang] Lower pack|unpack intrinsics

This patch adds lowering for the `pack` and `unpack`
intrinsics.

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

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D121823



Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: mleair <leairmark@gmail.com>
Co-authored-by: Kiran Chandramohan <kiran.chandramohan@arm.com>