[flang] Merge pull request flang-compiler/f18#118 from flang-compiler/pmk2

Better error recovery in the face of input that's not Fortran at all

Original-commit: flang-compiler/f18@7edf728444614f0eeda94c6b6dddd407a0cf380b
Reviewed-on: https://github.com/flang-compiler/f18/pull/118