Exhaustive when support for sealed classes #KT-7606 Fixed

When on sealed can use is (both for derived classes and objects) or just comparison (only for derived objects).
A pack of tests provided.

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

@@ -0,0 +1,11 @@
+sealed class Tree {
+    object Empty: Tree()
+    class Leaf(val x: Int): Tree()
+    class Node(val left: Tree, val right: Tree): Tree()
+
+    fun max(): Int = when(this) {
+        is Empty -> -1
+        is Leaf  -> <!DEBUG_INFO_SMARTCAST!>this<!>.x
+        is Node  -> <!DEBUG_INFO_SMARTCAST!>this<!>.left.max()
+    }
+}