Fix promote_typejoin_union with UnionAll

[nalimilan]

The current version returns a type which is narrower than what typejoin computes, which can make broadcast fail. Return Any until we develop code to compute the same result as typejoin, as this is only used for inference.

Fixes #38422.

@vtjnash @JeffBezanson This is the simplest fix I could find, but please feel free to suggest better ways to compute the actual type that typejoin would return.

[vtjnash]

I think the permissive version might be:

var = TypeVar(T.var.name)
T = UnionAll(var, promote_typejoin_union(T{var}))

but I'm not sure if that's necessarily worthwhile.

[nalimilan]

OK, let's merge this, and keep discussing the better approach.

@JeffBezanson Do you have an opinion?