Allow useEffect(fn, undefined) in react-hooks/exhaustive-deps. (#27525)

## Summary

There is a bug in the `react-hooks/exhaustive-deps` rule that forbids
the dependencies argument from being `undefined`. It triggers the error
that the dependency list is not an array literal. This makes sense in
pre ES5 strict-mode environments as undefined could be redefined, but
should not be a concern in today's JS environments.

**Justification:**
* The deps argument being undefined (for `useEffect` calls etc.) is a
valid use case for hooks that should re-run on every render.
* The deps argument being omitted is considered a valid use case by the
`exhaustive-deps` rule already.
* The TypeScript type definitions support passing `undefined` because
hooks are typed as `useEffect(effect: EffectCallback, deps?:
DependencyList): void;`.
* Since omitting an argument and passing `undefined` are considered
equivalent, this eslint rule should consider them as equivalent too.

Further, I accidentally forgot passing a dependency array to `useEffect`
in code that I shared on Twitter, and people started abusing me about
it. I'd like to create an eslint rule for my projects that requires me
to provide a dep argument in all cases (`undefined`, `[]` or the list of
dependencies) so that I can avoid such problems in the future. This
would also force me to always think about the dependencies instead of
accidentally forgetting them and my hook running on each render. In an
audit of my own codebase I had about 3% of hooks that I want to run on
each render, and adding an explicit `undefined` seems reasonable in
those situations.

It could be argued this could be an option or part of the
`exhaustive-deps` rule, but it's probably better to merge this PR, make
a release and see if my custom eslint rule gains traction in the future.

## How did you test this change?

* Added a test.
* `yarn test ESLintRuleExhaustiveDeps-test`
* Careful code inspection.

DiffTrain build for [d947c2f](d947c2f)

Author: kassens

compiled/facebook-www/REVISION

@@ -1 +1 @@
-77c4ac2ce88736bbdfe0b29008b5df931c2beb1e
+d947c2f1100dfd006fd91c6e9ed84d42ca7ab088

compiled/facebook-www/eslint-plugin-react-hooks.js

@@ -1856,7 +1856,8 @@ var ExhaustiveDeps = {
       var callback = node.arguments[callbackIndex];
       var reactiveHook = node.callee;
       var reactiveHookName = getNodeWithoutReactNamespace(reactiveHook).name;
-      var declaredDependenciesNode = node.arguments[callbackIndex + 1];
+      var maybeNode = node.arguments[callbackIndex + 1];
+      var declaredDependenciesNode = maybeNode && !(maybeNode.type === 'Identifier' && maybeNode.name === 'undefined') ? maybeNode : undefined;
       var isEffect = /Effect($|[^a-z])/g.test(reactiveHookName); // Check whether a callback is supplied. If there is no callback supplied
       // then the hook will not work and React will throw a TypeError.
       // So no need to check for dependency inclusion.