[Experiment] Reuse memo cache after interruption (#28878)

Adds an experimental feature flag to the implementation of useMemoCache,
the internal cache used by the React Compiler (Forget).

When enabled, instead of treating the cache as copy-on-write, like we do
with fibers, we share the same cache instance across all render
attempts, even if the component is interrupted before it commits.

If an update is interrupted, either because it suspended or because of
another update, we can reuse the memoized computations from the previous
attempt. We can do this because the React Compiler performs atomic
writes to the memo cache, i.e. it will not record the inputs to a
memoization without also recording its output.

This gives us a form of "resuming" within components and hooks.

This only works when updating a component that already mounted. It has
no impact during initial render, because the memo cache is stored on the
fiber, and since we have not implemented resuming for fibers, it's
always a fresh memo cache, anyway.

However, this alone is pretty useful — it happens whenever you update
the UI with fresh data after a mutation/action, which is extremely
common in a Suspense-driven (e.g. RSC or Relay) app.

So the impact of this feature is faster data mutations/actions (when the
React Compiler is used).