Enum path inference with `_Variant` syntax

I don't think using _Variant instead of _::Variant or .Variant really solves anything. Having it be ambiguous with a no-unused-variable-warning binding seems worse, to me.

To me, the blocker here has not been syntax, but the naming thing (which you did mention)

The syntax we could just pick one of the possible answers, if we really wanted.

But if .Yes could be AllowUnstable::Yes or Capturing::Yes or IsPattern::Yes, that's a more fundamental concern that's harder to resolve. I don't really know how to make progress on it.