Enable Objective-C ARC support automatically for Apple platforms

[cmcgee1024]

There is a build setting called "CLANG_ENABLE_OBJC_ARC" that should be enabled on Apple platforms, but not for any others since that supports doesn't exist in swift toolchains there. When building PIFs the client shouldn't need to be aware of this implementation detail.

Once this is in place then SwiftPM won't need the following code in the PIF Builder:

#if os(macOS)
        // Objective-C support only for macOS
        settings[.CLANG_ENABLE_OBJC_ARC] = "YES"
#endif

[jakepetroules]

We'll probably want to enable this by checking a target's isPackage flag in Settings construction, since we don't want to opt in Xcode projects as that would break backwards compatibility.