use union of overload signatures return types as inferred return type for unannotated impl

### Describe the Bug

```py
from typing import overload

@overload
def foo(i: str) -> list[object]: ... # Overload return type `list[object]` is not assignable to implementation return type `list[int]`
@overload
def foo(i: int) -> list[object]: ... # Overload return type `list[object]` is not assignable to implementation return type `list[int]`

def foo(i):
# bonus: reveal_type(i) should be `str | int`
return [1]
```

here we could infer `list[object]` and not see any error