[flang] Semantics checker for STOP and ERROR STOP statements - namespaces sorted

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

Original-commit: flang-compiler/f18@d608779668e1c4f328ebb02dcc21536b2c0fdf2c
Reviewed-on: https://github.com/flang-compiler/f18/pull/367
Tree-same-pre-rewrite: false