Fix destruction of generator running in fibers during shutdown (#15158)

The destructor of generators is a no-op when the generator is running in a fiber,
because the fiber may resume the

Fix destruction of generator running in fibers during shutdown (#15158)

The destructor of generators is a no-op when the generator is running in a fiber,
because the fiber may resume the generator. Normally the destructor
is not called in this case, but this can happen during shutdown.

We detect that a generator is running in a fiber with the
ZEND_GENERATOR_IN_FIBER flag.

This change fixes two cases not handled by this mechanism:

- The ZEND_GENERATOR_IN_FIBER flag was not added when resuming a "yield from $nonGenerator"

- When a generator that is running in a fiber has multiple children (aka multiple generators yielding from it), all of them could be considered to also run in a fiber (only one actually is), and could leak if not destroyed before shutdown.

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