[flang] Lower basic function with scalar integer/logical return value

This patch allows the lowring of simple empty function with a
scalar integer or logical return value.
The code in ConvertType.cpp is cleaned up as well. This file was landed
together with the initial flang push and lowering was still a prototype
at that time. Some more cleaning will come with follow up patches.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D119698



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