[stdlib] a better pointer as associated object id

Author: glessard

stdlib/public/core/StringGuts.swift

@@ -422,8 +422,9 @@ extension _StringGuts {
 
   private static var associationKey: UnsafeRawPointer {
     // We never dereference this, we just use this address as a unique key
-    // TODO: perhaps use the address of a metatype instead
-    unsafe UnsafeRawPointer(Builtin.addressof(&_swiftEmptyArrayStorage))
+    unsafe unsafeBitCast(
+      ObjectIdentifier(_StringGuts.self), to: UnsafeRawPointer.self
+    )
   }
 
   internal func getAssociatedStorage() -> __StringStorage? {