[flang] Define the named constants for the C characters with

special semantics in the ISO_C_BINDING intrinsic module.

Original-commit: flang-compiler/f18@bee546fdb22bd0a4f0a287e623fcb69ceafeb976
Reviewed-on: https://github.com/flang-compiler/f18/pull/660
Tree-same-pre-rewrite: false