[rcr] Re-export useMemoCache in top level React namespace (#31139)

In order to support using the compiler on versions of React prior to 19,
we need the ability to statically import `c` (aka useMemoCache) or
fallback to a polyfill supplied by `react-compiler-runtime` (note: this
is a separate npm package, not to be confused with
`react/compiler-runtime`, which is currently a part of react).

To do this we first need to re-export `useMemoCache` under the top level
React namespace again, which is additive and thus non-breaking. Doing so
allows `react-compiler-runtime` to statically either re-export
`React.__COMPILER_RUNTIME.c` or supply a polyfill, without the need for
a dynamic import which is finicky to support due to returning a promise.

In later PRs I will remove `react/compiler-runtime` and update the
compiler to emit imports to `react-compiler-runtime` instead.

packages/react/index.development.js

@@ -30,6 +30,7 @@ export type ChildrenArray<+T> = $ReadOnlyArray<ChildrenArray<T>> | T;
 // We can't use export * from in Flow for some reason.
 export {
  __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ __COMPILER_RUNTIME,
  Children,
  Component,
  Fragment,

packages/react/index.experimental.development.js

@@ -9,6 +9,7 @@
 
 export {
  __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ __COMPILER_RUNTIME,
  Children,
  Component,
  Fragment,

packages/react/index.experimental.js

@@ -9,6 +9,7 @@
 
 export {
  __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ __COMPILER_RUNTIME,
  Children,
  Component,
  Fragment,

packages/react/index.fb.js

@@ -9,6 +9,7 @@
 
 export {
  __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ __COMPILER_RUNTIME,
  act,
  cache,
  Children,

packages/react/index.js

@@ -31,6 +31,7 @@ export type ChildrenArray<+T> = $ReadOnlyArray<ChildrenArray<T>> | T;
 // We can't use export * from in Flow for some reason.
 export {
  __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ __COMPILER_RUNTIME,
  Children,
  Component,
  Fragment,

packages/react/src/ReactClient.js

@@ -63,6 +63,7 @@ import ReactSharedInternals from './ReactSharedInternalsClient';
 import {startTransition} from './ReactStartTransition';
 import {act} from './ReactAct';
 import {captureOwnerStack} from './ReactOwnerStack';
+import ReactCompilerRuntime from './ReactCompilerRuntime';
 
 const Children = {
  map,
@@ -109,6 +110,7 @@ export {
  isValidElement,
  ReactVersion as version,
  ReactSharedInternals as __CLIENT_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE,
+ ReactCompilerRuntime as __COMPILER_RUNTIME,
  // Concurrent Mode
  useTransition,
  startTransition,

packages/react/src/ReactCompilerRuntime.js

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import {useMemoCache} from './ReactHooks';
+
+export default {
+ c: useMemoCache,
+};