C++ Feature: Mutable union getters (#8852)

* generate mutable union accessors

* add test

* Revert "add test"

This reverts commit 45e352b18f93c9d59afef6498f9948b2dcb8c0a4.

* update file

* formatter got in the way

* merge conflicts

* updated genned code

* manually fix code gen bc I can't figure out why this file won't code gen

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Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>