New semantic analyzer: Refactor handling of is_inferred_def & co

There are several things that hold information about whether a type of variable should be inferred or not:

• RefExpr.is_new_def
• RefExpr.is_inferred_def
• Var.is_ready
• Var.is_inferred
• TypeChecker.is_definition()

Such a variety of things is partially caused by the fact that special definitions like named tuples etc. used to have a Var() to which the lvalue was pointing. With the new analyzer we don't have this, so it would be good to clean-up/refactor the logic around inference after we merge the two analyzers.

See also https://github.com/python/mypy/issues/6453

ファイルやテストは指定されていません。2つのアナライザーを統合した後、RefExpr.is_new_def、RefExpr.is_inferred_def、Var.is_ready、Var.is_inferred、TypeChecker.is_definition() を確認することから始めます。推論状態が一貫して扱われ、特殊な定義に関する古い Var の前提がなくなれば完了です。