[flang] Enable complex type in function lowering

This patch enables complex type in lowering.
It is tested on function return types.

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

Depends on D119698

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D119700



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