[flang] Merge pull request flang-compiler/f18#660 from flang-compiler/sjs-c-chars

Define the named constants for the C characters with special semantics

Original-commit: flang-compiler/f18@9c4ce244ad0447b923964431cf3b9f772fa03af0
Reviewed-on: https://github.com/flang-compiler/f18/pull/660