[flang] Merge pull request flang-compiler/f18#463 from flang-compiler/pmk-fix-brian

Fix bug with comment clipping on compiler directives

Original-commit: flang-compiler/f18@9307806d7ab8942d61294c97362a2031d1c4b0c5
Reviewed-on: https://github.com/flang-compiler/f18/pull/463