Bidirectional type inference with unions

This snippet currently type-checks:
```py
from typing import TypedDict, overload, reveal_type

class T(TypedDict):
  x: int


def f(x: T): ...

x: T | int = { "y": 1 }
reveal_type(x)
```

Bi-directional type-inference doesn't see through the union, so we infer `dict[Unknown | str, Unknown | int]`. We also don't end up validating the `TypedDict` constructor.