GeneratorDriverState and GeneratorState should be records

[chsienki]

It would be nice if GeneratorDriverState and GeneratorState were records. I don't think we've had many opportunities to dogfood records.

Originally posted by @RikkiGibson in #58800 (comment)

Would also be a big code simplification win.

[jcouv]

The reason we haven't used records much in roslyn is because we don't often rely on structural equality. So we didn't want to add all those methods to types when they aren't necessary to our scenarios.

[jcouv]

@RikkiGibson @chsienki I'm pushing back a bit on making those types records. What do you think? If you're convinced, we could close this issue.

[RikkiGibson]

I would like for us to at least measure the increase in code size, for example, before completely writing it off.

Basically, it concerns me a little that we so carefully designed records to work for such a variety of use cases, and then we feel that we don't want to use them in new code.