Attempt to fix Xcode bots after 4a0485be

A few macOS Xcode bots are seeing failures after commit 4a0485be (cmStandardLevelResolver: Avoid unnecessary flags, fix unset level logic, 2021-05-29):
https://open.cdash.org/testSummary.php?project=1&name=Module.WriteCompilerDetectionHeader&date=2021-10-05

It seems the cmIsOn() change could've changed the behaviour since now any non-ON values means false, while previously any non-OFF value meant ON.
Doesn't seem to be the case for nightlies, but might need to be changed to be behind the policy.

The iterator comparison change per my analysis shouldn't have changed anything besides clarifying how the code is intended to work. However, this seems the only possible cause for the nightly failures.