Emit a diagnostic when widening a singleton with `| Unknown`

[carljm]

We've mostly removed gradual-guarantee unioning with Unknown now; the one place in which we still do it is if an attribute is not annotated and is initialized with = None (or another non-Literal singleton type). It's unlikely that the intent is for the attribute to only ever be None, so in this case we widen to None | Unknown.

For those who want to keep dynamic types out of their code, it would be nice to get a diagnostic when this occurs, to prompt adding an annotation.

This could be part of #622, but I think there might be advantages to making it a distinct diagnostic, or at least offering additional context in this case.

[janlarres]

Would it be possible to infer the type from the first non-None assignment to a variable to avoid an explicit annotation, at least in some cases?

[carljm]

@janlarres If the "first non-None assignment" exists somewhere in methods of the class, then we already will find and include that assignment's type (and all other assignments in methods) into the overall type of the attribute, and we shouldn't widen with Unknown. (Though your comment led me to observe a bug; we shouldn't include Unknown in this case; working on a fix.)

Once that bug is fixed, the cases we are talking about here are where = None is the only information available to us in the class body, and any other assignment is external to the class definition. In this case it's not really even clear where we would look for "the first non-None assignment"; it could be anywhere in the codebase.