[red-knot] handle unions on the LHS of is_subtype_of (#13857)
## Summary
Just a drive-by change that occurred to me while I was looking at
`Type::is_subtype_of`: the existing pattern for unions on the *right
hand side*:
```rs
(ty, Type::Union(union)) => union
.elements(db)
.iter()
.any(|&elem_ty| ty.is_subtype_of(db, elem_ty)),
```
is not (generally) correct if the *left hand side* is a union.
## Test Plan
Added new test cases for `is_subtype_of` and `!is_subtype_of`
