[flang] Answer comments

* Support target label of label-do on all executable constructs as well as
  forall and where.
* Re-oder and rename related checks in resolve-labels.cc

Original-commit: flang-compiler/f18@e5ac8a1660511a7e48f0b2075f87ca1d8dbe23d9
Reviewed-on: https://github.com/flang-compiler/f18/pull/552
Tree-same-pre-rewrite: false