[DSLX] Pre-factoring IR converter, outstanding cleanup of visitation callbacks.
[DSLX] Pre-factoring IR converter, outstanding cleanup of visitation callbacks.

This was residual from the port from Python to C++ where C++ needed callbacks
to get back to the top level Python routine. Now can just have a member
function on the class responsible for everything.

PiperOrigin-RevId: 359299833

google/xls,google/xls,google/xls,google/xls,google/xls,google/xls