NSURL: Use the platform path style
NSURL: Use the platform path style

Not all platforms are POSIX.  Use the platform URL style when converting
the paths.  This was possible in CoreFoundation through the
`PLATFORM_PATH_STYLE` macro, but, unfortunately, that is unavailable to
us in Swift.  Emulate that and use it.

apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,e78l/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,parkera/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,e78l/swift-corelibs-foundation,parkera/swift-corelibs-foundation