Add symfony polyfill overrides

The purpose of the polyfill is to have pure PHP fallback implementations for when the PHP install is missing an extension or on an older version. Except in Debian we can simply replace them with the proper extensions.

For non-default extensions, a fourth field was added to the overrides format to indicate that the existing version constraint should be discarded since it doesn't make sense for things that follow a different versioning scheme. A future enhancement might allow using that field to specify a replacement version constraint.

See #911832 for some context.

This will also fix #821138 after src:symfony is rebuilt.