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/ExhaustiveWhenOnNullable.kt

@@ -0,0 +1,17 @@
+sealed class Sealed(val x: Int) {
+    object First: Sealed(12)
+    open class NonFirst(x: Int, val y: Int): Sealed(x) {
+        object Second: NonFirst(34, 2)
+        object Third: NonFirst(56, 3)
+    }
+}
+
+fun foo(s: Sealed?): Int {
+    return when(s) {
+        is Sealed.First -> 1
+        is Sealed.NonFirst -> 0
+        null -> -1
+        // no else required
+    }
+}
+