Failing test involving object rest/spread and clearScope().

This failing test case demonstrates a regression between 7.0.0-beta.38 and
7.0.0-beta.39 in the @babel/plugin-proposal-object-rest-spread package.

I distilled this test case from a larger configuration of plugins in my
application, one of which calls api.traverse.cache.clearScope(). Although
calling clearScope() is an uncommon thing for a plugin to do, it was a
reliable way to reproduce the problem. If I can find other reliable
reproductions, I'll push some additional failing tests to this PR.
Regardless of how common it is, clearing the scope cache should be a safe
operation that only slows down the transform (because scopes have to be
recreated and re-crawled). Crashing due to a spurious duplicate
declaration seems like a bug worth fixing.

My hunch is that [these two lines](eb38ea2b10/packages/babel-plugin-proposal-object-rest-spread/src/index.js (L75-L76))
(which were changed in `7.0.0-beta.39`) are not actually removing the
original rest element as a binding from the enclosing `Scope`, in certain
circumstances, so the new variable declaration ends up colliding with the
old (removed) binding.

Possibly related: #7304 (reported by @julien-f)

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/duplicate-decl-bug/input.js

@@ -0,0 +1,4 @@
+it("es7.objectRestSpread", () => {
+  let original = { a: 1, b: 2 };
+  let { ...copy } = original;
+});

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/duplicate-decl-bug/options.json

@@ -0,0 +1,7 @@
+{
+  "plugins": [
+    "./plugin-clear-scope",
+    "proposal-object-rest-spread",
+    "external-helpers"
+  ]
+}

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/duplicate-decl-bug/output.js

@@ -0,0 +1,7 @@
+it("es7.objectRestSpread", () => {
+  let original = {
+    a: 1,
+    b: 2
+  };
+  let copy = babelHelpers.objectWithoutProperties(original, []);
+});

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-rest/duplicate-decl-bug/plugin-clear-scope.js

@@ -0,0 +1,11 @@
+"use strict";
+exports.__esModule = true;
+exports.default = function (api) {
+  return {
+    visitor: {
+      Program: function () {
+        api.traverse.cache.clearScope();
+      }
+    }
+  };
+};