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/testData/codegen/box/reflection/constructors/classesWithoutConstructors.kt

@@ -18,6 +18,7 @@ fun box(): String {
     assertTrue(Interface::class.constructors.isEmpty())
     assertTrue(Obj::class.constructors.isEmpty())
     assertTrue(C.Companion::class.constructors.isEmpty())
+    assertTrue(object {}::class.constructors.isEmpty())
 
     return "OK"
 }

compiler/testData/codegen/box/reflection/constructors/primaryConstructor.kt

@@ -49,11 +49,13 @@ fun box(): String {
     val p4 = TwoSecondaries::class.primaryConstructor
     assertNull(p4)
 
-    assertNotNull(En::class.primaryConstructor) // TODO: maybe primaryConstructor should be null for enum classes
+    assertNotNull(En::class.primaryConstructor)
 
     assertNull(I::class.primaryConstructor)
     assertNull(O::class.primaryConstructor)
     assertNull(C.Companion::class.primaryConstructor)
 
+    assertNull(object {}::class.primaryConstructor)
+
     return "OK"
 }