[flang] Semantics checker for STOP and ERROR STOP statements - one more batch of post-review tweaks

Signed-off-by: Paul Osmialowski <pawel.osmialowski@arm.com>

Original-commit: flang-compiler/f18@f49ee0df050cce83cc5cdd4c04a3d16465ebeacc
Reviewed-on: https://github.com/flang-compiler/f18/pull/367
Tree-same-pre-rewrite: false