Use helper-builder-react-jsx inside plugin-transform-react-inline-elements (#6294)

* Use helper-builder-react-jsx inside plugin-transform-react-inline-elements.

This avoids duplicating the logic for converting jsx elements to plain JavaScript.

* Add a comment which explains the _jsx signature, [skip ci]
so it is a little bit easier to understand what all those .splice() calls do

packages/babel-helper-builder-react-jsx/README.md

@@ -13,6 +13,10 @@ type ElementState = {
 };
 
 require("babel-helper-builder-react-jsx")({
+  filter: function (element: JSXElement) {
+    // if returns false, the element isn't transformed
+  },
+
   pre: function (state: ElementState) {
     // called before building the element
   },