[flang] Merge pull request flang-compiler/f18#439 from flang-compiler/pmk1

Structure constructor semantics, part 1 of 2

Original-commit: flang-compiler/f18@56f3f7bd908ba36c9e92a8773ffaf26a5ac54094
Reviewed-on: https://github.com/flang-compiler/f18/pull/439