enforce parenthesis on ambigious types

```py
def guard(x: object, y: object) -> a is int & b is str if True else a is str & b is int: ...
```
The tricky part is that `ast` removes parens...

Start with the provided guard annotation and investigate how Python's ast handling removes parentheses before basedmypy processes the type expression. Determine which ambiguous type forms require enforced parentheses and how the example should be diagnosed; done means those forms are handled unambiguously.

Written by the indexing model from the issue text.