[flang] Semantics checker for STOP and ERROR STOP statements - better variable names

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

Original-commit: flang-compiler/f18@8b32caa287149e72834aad3cdc45473678b57605
Reviewed-on: https://github.com/flang-compiler/f18/pull/367
Tree-same-pre-rewrite: false