When you read a class instance's first machine word (e.g. via `unsafeBitCast` to `UnsafePointer<UInt>` and dereferencing), what does that leading word of the object header contain on a 64-bit Apple platform?
final class C { var x = 1 }
let c = C()
let word = unsafeBitCast(c, to: UnsafePointer<UInt>.self).pointee