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/regressions/6276/expected.js

@@ -0,0 +1,3 @@
+var test = babelHelpers.jsx(T, {
+  "default": " some string "
+});