[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.