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/namespace/input.js

@@ -0,0 +1,7 @@
+function AComponent () {
+  return <BComponent/>
+
+  function BComponent () {
+    return <n:CComponent />
+  }
+}

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

@@ -0,0 +1,9 @@
+var _n$CComponent;
+
+function AComponent() {
+  return <BComponent />;
+
+  function BComponent() {
+    return _n$CComponent || (_n$CComponent = <n:CComponent />);
+  }
+}