NSObject: use `_getSuperclass()` instead of a SPI
NSObject: use `_getSuperclass()` instead of a SPI

`_swift_class_getSuperclass` is a standard library internal interface.
Since we have `_getSuperclass()` available, use that rather than
promoting the private interface to SPI.

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