New semantic analyzer: Finish iterations after no names were added/modified

[ilevkivskyi]

Currently we defer reporting undefined names until we reach iteration limit. A better solution would be to track whether any new names were added, and if not, make one more iteration where report all errors.

Note that this will also allow a cleaner solution for #6412

[ilevkivskyi]

One thing to be careful with is placeholder nodes: if they are replaced with a real node, it should count as an addition, but if one placeholder replaces another one, the we should count it as a change only in the new one is becomes_typeinfo=True.

I also it is better to have tests for type variables with values restrictions and bounds, since we put Anys there, defer, and add the variable. This relies on reprocessing the type variable definition.