dillon1000/react

Fix missing return pointer assignment (#15700)

* Fix missing return pointer assignment

I found a bug using the fuzz tester that manifested as incorrect
ordering of children in the host tree, but whose root cause was a
missing `return` pointer assignment on a work-in-progress fiber.

Usually return pointers are set during reconciliation
(`reconcileChildFibers`) but this particular assignment happens inside
the custom reconciliation implementation used by Suspense boundaries.

I would not be surprised if there were similar bugs related to incorrect
return pointers. You're supposed to update the return pointer
whenever a work-in-progress fiber is created, but there's nothing in
the contract of the `createFiber` or `createWorkInProgress` function
that implies this. I propose that we update their signatures to accept
the return fiber as an argument. I will do this in a follow-up.

In this commit, I rearranged `updateSuspenseComponent` slightly so that
every call to `createWorkInProgress` or a `createFiber*` function is
immediately followed by a return pointer assignment.

I hardcoded the fuzz test case that surfaced the bug.

* Update all progressed children in list

`progressedPrimaryChild` is a list, not a single fiber. Need to iterate
through every child and update their return pointers.