[flang] Update PFTBuilder

This patch update the PFTBuilder to be able to lower
the construct present in semantics.

This is a building block for other lowering patches that will be posted soon.

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

Reviewed By: PeteSteinfeld, schweitz

Differential Revision: https://reviews.llvm.org/D120336



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