[flang] Fix for broken/degenerate forall case

Fix for broken/degenerate forall case where there is no assignment to an
array under the explicit iteration space. While this is a multiple
assignment, semantics only raises a warning.
The fix is to add a test that the explicit space has any sort of array
to be updated, and if not then the do_loop nest will not require a
terminator to forward array values to the next iteration.

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D128973



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