Warn for bad useEffect return value

Is this too restrictive? Not sure if you would want to do like

```js
useEffect(() => ref.current.style.color = 'red');
```

which would give a false positive here. We can always relax it to only warn on Promises if people complain.

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -295,8 +295,23 @@ function commitHookEffectList(
       if ((effect.tag & mountTag) !== NoHookEffect) {
         // Mount
         const create = effect.create;
-        const destroy = create();
-        effect.destroy = typeof destroy === 'function' ? destroy : null;
+        let destroy = create();
+        if (typeof destroy !== 'function') {
+          if (__DEV__ && destroy !== null && destroy !== undefined) {
+            warningWithoutStack(
+              false,
+              'useEffect function must return a cleanup function or nothing.%s%s',
+              typeof destroy.then === 'function'
+                ? ' Promises and useEffect(async () => ...) are not ' +
+                  'supported, but you can call an async function inside an ' +
+                  'effect.'
+                : '',
+              getStackByFiberInDevAndProd(finishedWork),
+            );
+          }
+          destroy = null;
+        }
+        effect.destroy = destroy;
       }
       effect = effect.next;
     } while (effect !== firstEffect);

packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js

@@ -51,4 +51,32 @@ describe('ReactHooks', () => {
     ]);
     expect(ReactTestRenderer).toHaveYielded(['Did commit: A, B']);
   });
+
+  it('warns for bad useEffect return values', () => {
+    const {useLayoutEffect} = React;
+    function App(props) {
+      useLayoutEffect(() => {
+        return props.return;
+      });
+      return null;
+    }
+    let root;
+
+    expect(() => {
+      root = ReactTestRenderer.create(<App return={17} />);
+    }).toWarnDev([
+      'Warning: useEffect function must return a cleanup function or ' +
+        'nothing.\n' +
+        '    in App (at **)',
+    ]);
+
+    expect(() => {
+      root.update(<App return={Promise.resolve()} />);
+    }).toWarnDev([
+      'Warning: useEffect function must return a cleanup function or ' +
+        'nothing. Promises and useEffect(async () => ...) are not supported, ' +
+        'but you can call an async function inside an effect.\n' +
+        '    in App (at **)',
+    ]);
+  });
 });