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/NonExhaustiveWhenWithAnyCase.kt

@@ -0,0 +1,15 @@
+sealed class Sealed {
+    object First: Sealed()
+    open class NonFirst: Sealed() {
+        object Second: NonFirst()
+        object Third: NonFirst()
+    }
+}
+
+fun foo(s: Sealed): Int {
+    return <!NO_ELSE_IN_WHEN!>when<!>(s) {
+        is Sealed.First -> 1
+        !is Any -> 0
+    }
+}
+