[flang] Semantics checker for STOP and ERROR STOP statements.

This commit introduces a new checker (StopChecker) for STOP
and ERROR STOP Fortran statements along with a test code.

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

Original-commit: flang-compiler/f18@c5541745628a8c0c4d252386675c36ea83a68006
Reviewed-on: https://github.com/flang-compiler/f18/pull/367
Tree-same-pre-rewrite: false