Foundation: extract cfSource from RunLoop._Source

Making `cfSource` a member within the structure prevents allowing
overriding of the `open` class as the type it returns cannot be
deserialised.  Because the member is not a stored property, extract it
to an extension which removes it from the vtable.  This allows the use
of the private type in an external context.

Author: compnerd

Sources/Foundation/RunLoop.swift

@@ -327,8 +327,6 @@ extension RunLoop {
     @available(*, deprecated, message: "For XCTest use only.")
     open class _Source: NSObject {
         fileprivate var _cfSourceStorage: AnyObject!
-        fileprivate var cfSource: CFRunLoopSource { unsafeBitCast(_cfSourceStorage, to: CFRunLoopSource.self) }
-        
 
         public init(order: Int = 0) {
             super.init()
@@ -410,3 +408,9 @@ extension RunLoop {
         }
     }
 }
+
+extension RunLoop._Source {
+    fileprivate var cfSource: CFRunLoopSource {
+      unsafeBitCast(_cfSourceStorage, to: CFRunLoopSource.self)
+    }
+}