[flang] Merge pull request flang-compiler/f18#1096 from flang-compiler/pmk-char

First draft: runtime API for CHARACTER

Original-commit: flang-compiler/f18@5b89dc7e516a86865737d7e69cde312d461e7d50
Reviewed-on: https://github.com/flang-compiler/f18/pull/1096
tree: efa9ad53013fc2890900e048230dab9566009704
  1. flang/