[flang] C1167 : Check for exit statments in do-concurrent

Addresses https://github.com/flang-compiler/f18/issues/288

Original-commit: flang-compiler/f18@2a99e1ea5404fce9fd88ea2c9710ec4c01975eb6
Reviewed-on: https://github.com/flang-compiler/f18/pull/345