[stdlib] a less public association key

Author: glessard

stdlib/public/core/StringGuts.swift

@@ -421,9 +421,10 @@ extension _StringGuts {
 extension _StringGuts {
 
   private static var associationKey: UnsafeRawPointer {
+    private struct AssociationKey {}
     // We never dereference this, we only use this address as a unique key
     unsafe unsafeBitCast(
-      ObjectIdentifier(__StringStorage.self),
+      ObjectIdentifier(AssociationKey.self),
       to: UnsafeRawPointer.self
     )
   }