Fix crashes on variadic unpacking in synthetic types (#21555)

Fixes https://github.com/python/mypy/issues/21237 (and similar crashes).

The problem is that we cannot do some of the type argument normalization
early in `typeanal.py` (since that will cause crashes on some
pathological recursive aliases), so we do it in `semanal_typeargs.py`
later. However, the mixed traverser visitor is somewhat incomplete (most
notably w.r.t. generated methods). I add (most of) the missing parts.