Add a warning for TARGET_OS_* compilation conditions

[j-f1]

This helps people avoid silent mistakes when migrating from Objective-C:

#if TARGET_OS_IOS
//  ^ warning: 'TARGET_OS_*' preprocessor macros are not available in Swift; use 'os(iOS)' instead
//  ^ fix-it: replace with 'os(iOS)'
print("I will never be executed, even on iOS!")
#endif

[j-f1]

@swift-ci please test

[ahoppen]

Looks good

[ahoppen]

Could you run swift-format on the changes? https://github.com/swiftlang/swift-syntax/blob/main/CONTRIBUTING.md#formatting

[j-f1]

@swift-ci please test

[ahoppen]

@swift-ci Please test Windows