[mypyc] Borrow final attributes more aggressively

[JukkaL]

Final attributes can't be rebound after initialization, so we can borrow them more aggressively than regular attributes, as long as the base object is kept alive for the duration of the borrowing. This allows skipping some incref/decref operations, and it adds significantly more opportunities for borrowing on free-threaded builds, in particular, as borrowing non-final attributes is very restricted.

For example, the attribute x can only be safely borrowed if it's Final, since otherwise foo could assign to the attribute and free the old value:

def func(o: C) -> None:
    foo(o.x)

There are some subtleties in the implementation. Here are the main things that required extra care:

• We don't allow borrowed values to escape from conditionally executed code paths (e.g. conditional expressions, comprehensions).
• If a local variable can be modified with an assignment expression, we restrict borrowing based on that variable.
• We can only borrow for a longer duration if the attribute value doesn't depend on a subexpression with a smaller borrow scope.
• Lambda expressions generate a complete separate expression and borrowing scope.

I used coding agent assist, especially for tests, but created the implementation is short, individually reviewed increments.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅