Exhaustive when on sealed trees implemented #KT-13130 Fixed

Also #KT-13227 Fixed

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

@@ -0,0 +1,23 @@
+sealed class Stmt
+
+class ForStmt : Stmt()
+
+sealed class Expr : Stmt() {
+    object BinExpr : Expr()
+}
+
+fun test(x: Stmt): String =
+        when (x) {
+            is Expr -> "expr"
+            is Stmt -> "stmt"
+        }
+
+fun test2(x: Stmt): String =
+        <!NO_ELSE_IN_WHEN!>when<!> (x) {
+            is Expr -> "expr"
+        }
+
+fun test3(x: Expr): String =
+        when (x) {
+            is Stmt -> "stmt"
+        }