[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        |
please don't change the visibility (making it protected in the other loaders was a mistake)