Manipulation: Break reference copies in cloned properties

Fixes #15104

```
   raw     gz Compared to 1.x-master @ d4a998f62fde4b9347b99571a7d000b53731909a
   +95    +60 dist/jquery.js                                                   
   +22    +12 dist/jquery.min.js
```
`knownClient` was a test failure investigation dead-end that turned out to be otherwise convenient. If no one objects to them, these changes will need a cherry-pick into `master`.