Update AST helper generation

[systay]

Description

This PR removes the visitorgen component that used to create the Vitess Rewriter, and adds asthelpergen that generates both the rewriter, but also deep-clone methods for all AST types.

The Gen4 planner tries join in both directions. Given the following query:

SELECT a.col, b.col FROM A JOIN B ON a.id = b.id

The new planner will try to first plan the A side on the left, and the send data from A to B as params, and then will try the other variation. The two versions would be:

-- ********** A JOIN B
-- A side
SELECT a.col FROM A
-- B side
SELECT b.col FROM B WHERE ? = b.id



-- ********** B JOIN A
-- B side
SELECT b.col FROM B
-- B side
SELECT b.col FROM A WHERE a.id = ?

As can be seen in these examples, we need to be able to clone the original predicate and then change it in two different ways to test both alternatives.

Related Issue(s)

#7280

Checklist

• Should this PR be backported? - Absolutely not
• Tests were added or are not required - Lots of tests added
• Documentation was added or is not required - No docs needed. Internal change only

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving

[vmg]

Looks good to me! We spent quite a bit of time reviewing this together. ✨

[GuptaManan100]

Do we need a comment for an exported function?

[vmg]

@systay I now see where the performance regression in cd7c4b3 comes from -- you're actually allocating closures now! By removing the extra argument to every replacement function, all the visitor callbacks that were previously pure functions are becoming closures, which require an allocation even if they're not being called. Definitely a performance regression.

[harshit-gangal]

Really nice work.

[harshit-gangal]

looks like they are not used in the code.

[systay]

the comment is not lying. these consts produce code that we had to do extra checks in the generator code to avoid. They are here to mimic sqlparser types such as ReadOnly (https://github.com/vitessio/vitess/blob/master/go/vt/sqlparser/constants.go#L254)

[harshit-gangal]

nit: error is reserved word, better to name as err