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/jsx-in-prop/input.js

@@ -0,0 +1,3 @@
+function C() {
+  return <div attr=<span /> />;
+}

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/jsx-in-prop/output.js

@@ -0,0 +1,5 @@
+var _div;
+
+function C() {
+  return _div || (_div = <div attr=<span /> />);
+}