[flang] Semantics checker for STOP and ERROR STOP statements - post review changes

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

Original-commit: flang-compiler/f18@08b564832c8a2439f35698d55f1d2a4482e6a54d
Reviewed-on: https://github.com/flang-compiler/f18/pull/367
Tree-same-pre-rewrite: false