Get rid of Optional when assigning Any in an is None branch

[msullivan]

As a special case, when assigning Any to a variable with a
declared Optional type that has been narrowed to None,
replace all the Nones in the declared Union type with Any.

The result of this is:

def f():...
def g(x: Optional[int]) -> int:
    if x is None:
        x = f()
        reveal_type(x)   # Union[int, Any]  # <---- this is new (was Optional[int])
    reveal_type(x)  # Union[int, Any]  # was also Optional[int]
    return x  # Ok  # was an error

Fixes #3526.

[ilevkivskyi]

Finally this old issue will be fixed! :-)

[JukkaL]

Great to have this fixed! This has been a pain point for a long time.