[flang] More responses to comments in the pull request. The big change was to

implement an enum class to indicate whether the statement being checked
is a CYCLE or EXIT statement.  This change rippled through a few
interfaces, resulting in cleaner, more readable code.  Thanks for the
tip, Tim!

Original-commit: flang-compiler/f18@e167c3d39fa95c93f402a0bfdf916dc10dbd80e1
Reviewed-on: https://github.com/flang-compiler/f18/pull/756
Tree-same-pre-rewrite: false