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-plugin-transform-react-inline-elements/package.json

@@ -8,6 +8,9 @@
   "keywords": [
     "babel-plugin"
   ],
+  "dependencies": {
+    "babel-helper-builder-react-jsx": "7.0.0-beta.1"
+  },
   "devDependencies": {
     "babel-helper-plugin-test-runner": "7.0.0-beta.1"
   }