Do not serialize constructors for anonymous objects

The only client of this data is reflection, and since anonymous objects
do not have constructors in the source code, they shouldn't in
reflection as well

 #KT-20442 Fixed

compiler/serialization/src/org/jetbrains/kotlin/serialization/DescriptorSerializer.kt

@@ -89,7 +89,7 @@ class DescriptorSerializer private constructor(
             }
         }
 
-        if (classDescriptor.kind != ClassKind.ENUM_ENTRY) {
+        if (!DescriptorUtils.isAnonymousObject(classDescriptor) && classDescriptor.kind != ClassKind.ENUM_ENTRY) {
             for (descriptor in classDescriptor.constructors) {
                 builder.addConstructor(constructorProto(descriptor))
             }