[flang] Fix issue 392, improve the error message when a DO loop is

terminated by a statement other than END DO or CONTINUE.

Original-commit: flang-compiler/f18@0a9280eab16394b578d8b1eae76e3a679b697770
Reviewed-on: https://github.com/flang-compiler/f18/pull/398
Tree-same-pre-rewrite: false