Provide a diagnostic when @Serializable annotation is missing on enum.

It is required when enum members contain some special annotations which should be recorded in descriptor.
Due to some recursive/lazy resolve problems, compiler plugin frontend can't infer @Serializable on enum automatically, therefore, we ask a user to explicitly provide it.

Also fix ir tests

plugins/kotlin-serialization/kotlin-serialization-compiler/testData/codegen/Basic.ir.txt

@@ -962,4 +962,4 @@ public final class User : java/lang/Object {
     public final java.lang.String getFirstName()
 
     public final java.lang.String getLastName()
-}
+}