Lower bound must be a subtype of the upper bound.

The change in CommonSupertypes:

 We used to say that commonSupertype(Inv<A>, Inv<B>) = Inv<in Intersect(A, B)). This is counter-intuitive, should be Inv<out commonSupertype(A, B)>

compiler/testData/type-checker-test.kt

@@ -36,4 +36,8 @@ open class ArrayList<E>() : Any, AbstractList<E?>, List<E?>
 fun f() : Unit {}
 fun f(a : Int) : Int {a}
 fun f(a : Float, b : Int) : Float {a}
-fun f<T>(a : Float) : T {a}
+fun f<T>(a : Float) : T {a}
+
+trait Parent
+trait A: Parent
+trait B: Parent