Better representation of JSX in AST

[cknitt]

We should improve the JSX representation in the AST before implementing JSX preserve mode (#6197).

[nojaf]

So, my initial thoughts on this would be:

• Extend expression_desc with a new type Pexp_jsx_element.
• Update the parser to construct these new elements.
• Map correct to tast. After that happen, nothing should impact I would assume?
• Deal with tooling
• Deal with ppx (Would need some pointers here)

[cristianoc]

A first step could be to see how a few examples are currently mapped to the AST, and where is the code that deals with the output of this mapping.
Then find a more direct representation of the same information and modify the above code. It might be that a new expression desc, such as Pexp_jsx_element is completely processed by the ppx before it ever reaches the type checker, so is never used by later phased of the compiler (a bunch of places where that expression will never reach).
So I guess the first step is finding out what the current code path are.

[zth]

Like @cristianoc says, I would also guess that the type checker won't have to care about JSX given that it's transformed to and type checked as regular function calls.