[flang] Enable scalar real type in lowering

This patch enables scalar real 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/D119699



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