[Debug] Fixed ClassNotFoundFatalErrorHandler on windows.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT

Because `$path` is `realpath`'ed. So we have to use `DIRECTORY_SEPARATOR`
instead of `/` because the default directory separator on windows is
`\`. And so the `str_replace` can not work and can lead to a fatal error
because class could already be loaded. For example, the `Response` class
in symfony full stack is in the `bootstrap.php.cache`.


ping @nicolas-grekas