Cleanup `codegen_fn_attrs` The `match` control flow construct has been stable since 1.0, we should use it here. Sorry for the hard to review diff, I did try to at least split it into two commits. But looking at before-after side-by-side (instead of whatever github is doing) is probably the easiest way to make sure that I didn't forget about anything. On top of #109088, you can wait for that