Detect impossible condition caused by user defined __bool__

Eg with the following:

```python
from typing import *

class Foo:
 def __bool__(self) -> Literal[True]:
 return True

def some_foo() -> Foo:
 ...

def throw_error():
 raise Exception


if __name__ == '__main__':
 print(some_foo() or throw_error())
 # ^^^^^^^^^^^^^ impossible condition, unreachable
```

mypy should be able to detect that the call to throw_error is unreachable