Improve LazyClassDescriptor#getDefaultObjectDescriptor()

To cover the case when default object's name is not unique (redeclaration)
(Calling getClassifier is wrong for this case)

compiler/testData/diagnostics/tests/classObjects/classObjectRedeclaration.kt

@@ -0,0 +1,19 @@
+package test
+
+class A {
+    object <!REDECLARATION!>Default<!>
+
+    <!REDECLARATION!>class object<!>
+}
+
+class B {
+    class object <!REDECLARATION!>Named<!>
+
+    object <!REDECLARATION!>Named<!>
+}
+
+class C {
+    class <!REDECLARATION!>Named<!>
+
+    class object <!REDECLARATION!>Named<!>
+}