Replace 'class object' with 'default object' in renderers and test data

Includes changes to decompiled text
Old syntax is used in builtins and project code for now

compiler/testData/serialization/builtinsSerializer/annotationTargets.kt

@@ -32,6 +32,6 @@ class C1 [anno("constructor")] () {
     anno("nested class")
     class C2
 
-    anno("class object")
-    class object {}
+    anno("default object")
+    default object {}
 }

compiler/testData/serialization/builtinsSerializer/annotationTargets.txt

@@ -16,7 +16,7 @@ test.anno(x = "top level class": kotlin.String) internal final class C1 {
         public constructor C2()
     }
 
-    test.anno(x = "class object": kotlin.String) internal class object Default {
+    test.anno(x = "default object": kotlin.String) internal default object Default {
         private constructor Default()
     }
 }