TypedDict constructor validation doesn't run for unions

We show no diagnostics for:
```python
from typing import TypedDict

class Foo(TypedDict):
    a: int

class Bar(TypedDict):
    a: int

def f(T: type[Foo | Bar]):
    T(a="baz")
```

https://play.ty.dev/4d97ff89-b725-4ef3-918a-aeb75d817bae