[flang] Lower general forall statement

This patch lowers general forall statements. The forall
are lowered to nested loops.

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

Depends on D121385

Reviewed By: PeteSteinfeld, schweitz

Differential Revision: https://reviews.llvm.org/D121386



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