f-strings with unformattable values can break inferred literals in overloads

```py
from typing import overload

@overload
def f(): ...

@overload
def f(s: str, a: True = ...): ...

def f(s="", a=True): ...

v: None

f(f"{v}", True)
```

Start by running the Python reproducer with basedmypy and inspect the checker paths for f-string inference and overload resolution. Determine the intended inferred type for the formatted value and verify that the overload result no longer changes incorrectly for an unformattable value.

Written by the indexing model from the issue text.