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/constructor/output.js

@@ -1,7 +1,7 @@
+var _Foo;
+
 var Foo = require("Foo");
 
-var _ref = <Foo />;
-
 function render() {
-  return _ref;
+  return _Foo || (_Foo = <Foo />);
 }