Refactor: process mentions of class object

Rename usages that refer to "default object" concept now
Test data file names are left as is

compiler/testData/diagnostics/tests/regressions/kt2956.kt

@@ -1 +1 @@
-val a: String = <!NO_CLASS_OBJECT!>Nothing<!>
+val a: String = <!NO_DEFAULT_OBJECT!>Nothing<!>

compiler/testData/diagnostics/tests/regressions/kt4827.kt

@@ -11,6 +11,6 @@ class C {
 }
 
 fun f() {
-    <!NO_CLASS_OBJECT, FUNCTION_EXPECTED!>TestInterface<!>()
+    <!NO_DEFAULT_OBJECT, FUNCTION_EXPECTED!>TestInterface<!>()
     C.<!UNRESOLVED_REFERENCE!>I<!>()
 }