[flang] Lower sum intrinsic

This patch enables the lowering of the `sum` intrinsic. It adds
also infrastructure to deal with optional arguments in intrinsics and
implied loops.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121221



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