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/WhenOnEmptySealed.kt

@@ -0,0 +1,11 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
+sealed class Sealed {
+
+}
+
+fun foo(s: Sealed): Int {
+    return <!NO_ELSE_IN_WHEN!>when<!>(s) {
+        // We do not return anything, so else branch must be here
+    }
+}
+