Export php_date_get_interface_ce() for extension use

[jmikola]

@derickr: We spoke about this earlier in mongodb/mongo-php-driver#336 (comment):

Alternatively, do we need to use zend_fetch_class() and resolve the interface name string to a class entry?

That sounds kinda crappy... I'd rather export the interface in PHP (from PHP 7.1).

[Fleshgrinder]

Does this allow userland implementation of the DateTimeInterface? Note that userland classes can never fulfill the contract because they have no way of implementing the operator overloading that both DateTime and DateTimeImmutable provide.

[derickr]

This probably shouldn't be part of the patch.

[jmikola]

Not sure how that got in there. Should be removed now, though!

[derickr]

No, it's just for internal extensions that need to match against either DateTime or DateTimeImmutable.

[nikic]

Merged as 7aaa310. Thanks!