Functions with type variable value restriction lose some AST information

There are a bunch of mypy AST attributes that mypy.treetransform.TransformVisitor does not propagate when copying AST nodes. This causes issues in functions using a type variable restriction, since function bodies of such functions are transformed using this visitor before type checking. Based on a user report, I noticed that async-related flags aren't propagated. I also noticed a few others, including dataclass transform state. It's not clear if all of these are causing actual issues, but some of them might.

We should go through all AST nodes and double check that all relevant state is being copied. Not sure if there is an easy way to test exhaustively that we don't miss any attributes as AST attributes are added.

Start with mypy.treetransform.TransformVisitor and compare the attributes it copies against the state on all AST nodes, especially async-related flags and dataclass transform state. Check existing coverage for functions using restricted type variables, then add tests that confirm transformed nodes retain relevant state; done means relevant AST state is consistently preserved.