Exhaustive when on sealed trees implemented #KT-13130 Fixed

Also #KT-13227 Fixed

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

@@ -0,0 +1,34 @@
+sealed class Base {
+    sealed class A : Base() {
+        object A1 : A()
+        sealed class A2 : A()
+    }
+    sealed class B : Base() {
+        sealed class B1 : B()
+        object B2 : B()
+    }
+
+    fun foo() = when (this) {
+        is A -> 1
+        is B.B1 -> 2
+        B.B2 -> 3
+        // No else required
+    }
+
+    fun bar() = <!NO_ELSE_IN_WHEN!>when<!> (this) {
+        is A -> 1
+        is B.B1 -> 2
+    }
+
+    fun baz() = when (this) {
+        is A -> 1
+        B.B2 -> 3
+        // No else required (no possible B1 instances)
+    }
+
+    fun negated() = when (this) {
+        !is A -> 1
+        A.A1 -> 2
+        is A.A2 -> 3
+    }
+}