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/ExhaustiveWhenOnNestedSealed.kt

@@ -0,0 +1,20 @@
+sealed class Sealed {
+    object First: Sealed()
+    sealed class NonFirst {
+        object Second: NonFirst()
+        object Third: NonFirst()
+        object Fourth: Sealed()
+    }    
+}
+
+fun foo(s: Sealed, nf: Sealed.NonFirst): Int {
+    val si = when(s) {
+        Sealed.First -> 1
+        Sealed.NonFirst.Fourth -> 4
+    }
+    val nfi = when(nf) {
+        Sealed.NonFirst.Second -> 2
+        Sealed.NonFirst.Third -> 3
+    }
+    return si + nfi
+}