[HttpFoundation] Split getClientIp into two methods for better flexibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | Locally: NO (HttpFoundation tests passes, but all Symfony tests fails because of my local setup). Travis : YES
| Fixed tickets | #7349
| License       | MIT
| Doc PR        | none (yet?)

Split the ``Request::getClientIp`` method in two to allow better overriding flexibility. See #7349 for more information.