Exhaustive when on sealed trees implemented #KT-13130 Fixed

Also #KT-13227 Fixed

compiler/testData/diagnostics/tests/sealed/ExhaustiveOnTriangleStar.kt

@@ -0,0 +1,24 @@
+sealed class A
+sealed class B : A()
+
+class C : B()
+class D : B()
+
+fun test(a: A): Any {
+    return when (a) {
+        is C -> ""
+        is D -> ""
+    }
+}
+
+fun test2(a: A): Any {
+    return when (a) {
+        is B -> ""
+    }
+}
+
+fun test3(a: A): Any {
+    return <!NO_ELSE_IN_WHEN!>when<!> (a) {
+        is D -> ""
+    }
+}