[Flang] Lower the FailImage Statement

Lowering of FailImage statement generates a runtime call and the
unreachable operation. The unreachable operation cannot terminate
a structured operation like the IF operation, hence  mark as
unstructured.

Note: This patch is part of upstreaming code from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D124520



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