[flang] Merge pull request flang-compiler/f18#720 from schweitzpgi/resurrect-flat-form

re-add files from old FIR code

Original-commit: flang-compiler/f18@5bf0827c20f4b9be3b49c1fd28bb14871a3ed9ad
Reviewed-on: https://github.com/flang-compiler/f18/pull/720