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/ExhaustiveWhenDoubleInner.kt

@@ -0,0 +1,16 @@
+sealed class Sealed() {
+    object First: Sealed()
+    open class NonFirst: Sealed() {
+        object Second: NonFirst()
+        object Third: NonFirst()
+        // It's ALLOWED to inherit Sealed also from here
+        object Fourth: Sealed()
+    }    
+}
+
+fun foo(s: Sealed) = when(s) {
+    Sealed.First -> 1
+    is Sealed.NonFirst -> 2
+    Sealed.NonFirst.Fourth -> 4
+    // no else required
+}