Lazily initialize and cache constant JSX elements (#12967)

Co-authored-by: Justin Ridgewell <justin@ridgewell.name>

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/children/output.js

@@ -1,9 +1,9 @@
-var _ref = <span />;
+var _span;
 
 var Foo = React.createClass({
   render: function () {
     return <div className={this.props.className}>
-      {_ref}
+      {_span || (_span = <span />)}
     </div>;
   }
 });