Fix assertion failure in generator dtor

[arnaud-lb]

Fixes GH-15866. Related: GH-15158, GH-10462.

I don't remember why I assumed that a generator with the ZEND_GENERATOR_IN_FIBER flag (implying the generator is running) could only be dtor during shutdown, but this assumption was wrong. It can happen during GC, of course.

Unfortunately it means that we can not cache the result of check_node_running_in_fiber() anymore, so we may traverse the tree more than once. However we only ever call this function when collecting a Fiber, which I expect to be rare.

This is already the 3rd follow up of GH-10462, so I considered this idea again, but since we can start fibers in destructors now, this is not trivial.

[arnaud-lb]

I assume the ABI break is because of the removal of ZEND_GENERATOR_DTOR_VISITED, but this symbol is not in any release yet.

[cmb69]

I assume the ABI break is because of the removal of ZEND_GENERATOR_DTOR_VISITED, but this symbol is not in any release yet.

The labeler is dumb. If there is no ABI break, just remove the label (if ZEND_GENERATOR_DTOR_VISITED was already in PHP-8.3, the label would be a nice reminder to document the removal in UPGRADING.INTERNALS).

[bwoebi]

I'm approving it because the changes are definitely right, just without caching.
Caching would be nice, but I suppose we can defer that until it actually becomes a problem?! (As far as I'm aware it hasn't been one.)