A passive effect's cleanup function may throw after an unmount. Prior to this commit, such an error would be ignored. (React would not notify any error boundaries.) After this commit, React's behavior varies depending on which reconciler fork is being used.
For the old reconciler, React will call componentDidCatch for the nearest unmounted error boundary (if there is one). If there are no unmounted error boundaries, React will still swallow the error because the return pointer has been disconnected, so the normal error handling logic does not know how to traverse the tree to find the nearest still-mounted ancestor.
For the new reconciler, React will skip any unmounted boundaries and look for a still-mounted boundary. If one is found, it will call getDerivedStateFromError and/or componentDidCatch (depending on the type of boundary).
Tests have been added for both reconciler variants for now.
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