A more general case of #8656

After running an app on iOS and then cleaning, there are still the following artifacts:

	ios/Flutter/App.framework/
	ios/Flutter/Flutter.framework/
	ios/Flutter/Generated.xcconfig
	ios/Flutter/app.flx
	ios/ServiceDefinitions.json

Ideally, we'd produce those artifacts in the build/ directory, but in absence of that, we should at least clean them out.