Sealed code generation: sealed is considered abstract, correct serialization

A pair of tests provided.

compiler/testData/codegen/box/sealed/objects.kt

@@ -0,0 +1,11 @@
+sealed class Season {
+    object Warm: Season()
+    object Cold: Season()
+}
+
+fun foo(): Season = Season.Warm
+
+fun box() = when(foo()) {
+    Season.Warm -> "OK"
+    Season.Cold -> "Fail: Cold, should be Warm"
+}