Union of a class with Any base and None is sometimes None

[JukkaL]

Without --strict-optional, the second reveal_type produces an incorrect result:

from typing import Any, Union, TypeVar

class C(Any):
    pass

T = TypeVar('T')
S = TypeVar('S')

def f(x: T, y: S) -> Union[T, S]:
    pass

reveal_type(f(C(), None))  # t.C
reveal_type(f(None, C()))  # builtins.None   <--- wrong

With --strict-optional, both of them are wrong:

...   # as above

reveal_type(f(C(), None))  # builtins.None   <--- wrong
reveal_type(f(None, C()))  # builtins.None   <--- wrong

This seems to cause problems in real-world code together with python/typeshed@6008b9d, so this is high priority.

[JukkaL]

This looks very similar to the issue fixed by #2197. Maybe I'll just finish up that PR.