[flang] Fixes several bugs relating to initialization expressions. An (#1493)

Fixes several bugs relating to initialization expressions. An
initialization expression has no access to dynamic resources like the
stack or the heap. It must reduce to a relocatable expression that the
loader can complete at runtime.

Adds regression test.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128380



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