KT-2438 Prohibit inner classes with the same name

Rewrite checkClassObjectInnerClassNames() to also
check for name clashes between inner classes of a
single class
 #KT-2438 Fixed

compiler/testData/diagnostics/tests/redeclarations/kt2438.kt

@@ -0,0 +1,21 @@
+//KT-2438 Prohibit inner classes with the same name
+
+package kt2438
+
+class B {
+    class <!REDECLARATION!>C<!>
+    class <!REDECLARATION!>C<!>
+}
+
+
+
+class A {
+    class <!REDECLARATION!>B<!>
+    
+    class object {
+        class <!REDECLARATION!>B<!>
+        class <!REDECLARATION!>B<!>
+    }
+    
+    class <!REDECLARATION!>B<!>
+}