New semantic analyzer: placeholder nodes from self.x assignment

[JukkaL]

This test case generates unexpected output:

class C:
    def f(self) -> None:
        self.x: D = D()  # Expression has type "D", variable has type "int"
        self.x: int
        class D: pass

reveal_type(C().x)  # int

The first assignment should take precedence, even though it contains a forward reference. We could probably support this by inserting a placeholder node into the class symbol table.

Follow-up to #6336.