[flang] Merge pull request flang-compiler/f18#1099 from flang-compiler/pmk-assign

Complete semantics for ASSIGN

Original-commit: flang-compiler/f18@b539ac9a908d5c9f7232d4635ad543c23947c96f
Reviewed-on: https://github.com/flang-compiler/f18/pull/1099
tree: 5998d3324a81e459688b90c53ba1df51a75994f6
  1. flang/