[ClassLoader] Add a __call() method to XcacheClassLoader

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11733 
| License       | MIT
| Doc PR        |
this cannot be changed (we cannot turn a protected property into private because it is a BC break)