Use type context from previous definition when renaming

Maybe we code like this shouldn't generate an error when using --allow-redefinition:

x: List[int] = []
# do something with x
x = []  # Error: need annotation

The reason for the error is that the second assignment creates a completely independent variable, so the type of the original definition has no effect.

There are still some open issues:

• Using the context can generate false positives, in case the correct type would be something different for the second definition. I don't know how often this would be an issue.
• Propagating the type context is somewhat tricky since the name of the previous definition is not trivial to calculate. The rules for renaming depend on the scope (local/class/global).
• If/when we support more general redefinitions (not just within a single block), it may be unclear when we should propagate context. It may be best to first to make redefinitions more general and then reconsider this issue.

Follow-up to #6197.

まず、フォローアップ issue #6197 と、この issue で説明されている再定義の動作を読んでください。提供されている --allow-redefinition の例を使って、列挙されている誤検知と名前変更に関する懸念を含め、スコープ間で型コンテキストをどのように扱うべきかを判断してください。実装前に意図した動作とスコープが確定していれば完了です。