[flang] Lower complex constant

Add ability to lower complex constant.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D120402



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