DateTimeZone::getOffset() now accepts a DateTimeInterface

Fixes https://bugs.php.net/bug.php?id=68062

This is a BC break, since the arguments are now declared using OBJ_ARG_INFO instead of plain ARG_INFO.

Should it be backported to 5.*, it would have to be kept with ARG_INFO.
If I don't do this, PHP will throw a catchable fatal error at the first argument of the wrong type, so the test won't be able to run all the checks