[flang] Add non standard feature for labeled do not ending with END DO or CONTINUE

* The warning was already here, this commit only refactors things so that
 it can be controled with -Mstandard.
* Also makes the warning point to the do-stmt and adds a note to the warning
  pointing to the statements where it ends.

Original-commit: flang-compiler/f18@11e1eb6edd4a88b7d6517ecfc3378e95757a3a24
Reviewed-on: https://github.com/flang-compiler/f18/pull/552
Tree-same-pre-rewrite: false