Make Warns = [Located DriverMessage]
This is another take on !10450 (closed), delaying the creation of message envelopes.
The unrecognised flag warning is unelegant, but avoids resolving !23402 for now: i.e. we do check whether flag is currently on, and do not produce the diagnostic. However, if the final state is as with -Wno-unrecognised-warnings
, none will be shown. Yet, no tests fail due that, so maybe it worked so already.