Bring the Symfony\Component\ClassLoader\ApcClassLoader into core and undeprecate

Composer introduced its own APCu integration in 1.3.0. Would it make sense to defer to this when available and not already enabled?

Alternately, if the same decoration pattern is used, I think the Composer handling may need to be disabled to prevent double-caching.

https://getcomposer.org/doc/articles/autoloader-optimization.md#optimiza...
https://github.com/composer/composer/commit/6d4e60b9914e5514c49f4b19ee2b...

Need to deal also with Symfony\Component\ClassLoader\WinCacheClassLoader, which deprecation still needs to be added to the silenced deprecations list in #2938369: The Symfony\Component\ClassLoader\WinCacheClassLoader class is deprecated.

Related #2938369: The Symfony\Component\ClassLoader\WinCacheClassLoader class is deprecated is fixed, back to active.

Two patches:

a) Use Composer's built in APCu caching if available, but fall back to Symfony decorators; overrides the prefix used by composer since it doesn't accommodate all the items that Drupal's prefix includes. Use of symfony/class-loader could be removed later when updating to Symfony 4.

b) Remove use of Symfony decorators and only use Composer's caching. Keeps settings option to be able to disable APCu caching, and class loader can still be decorated within settings.php.
symfony/class-loader isn't removed as a dependency from composer.json because an existing site could rely on it without declaring it as a dependency (either because it's using the zip archive of core, or doesn't include it in the site's composer.json), and the example code in default.settings.php uses \Symfony\Component\ClassLoader\ApcClassLoader.
The only references in core to either wincache or xcache that I could see were in DrupalKernel and rebuild.php. A wincache / xcache contrib module could provide both a service (e.g. cache.backend.wincache) and the necessary settings snippet to make use of the cache for classloader lookups.

Classloader has been removed in sf4 and d9 needs workaround to remove it at all or better replace it with related issue

I believe this is a duplicate of #2704571: Add an APCu classloader with a single entry