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/test/fixtures/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["external-helpers", "syntax-jsx", "transform-react-inline-elements", "transform-react-jsx"]
+}