[FIR] Add diagnostic CONFLICTING_OVERLOADS & REDECLARATION

compiler/testData/diagnostics/tests/classObjects/ClassObjects.fir.kt

@@ -2,10 +2,10 @@
 package Jet86
 
 class A {
-  companion object {
+  companion <!REDECLARATION!>object<!> {
     val x = 1
   }
-  companion <!MANY_COMPANION_OBJECTS!>object<!> {
+  companion <!MANY_COMPANION_OBJECTS, REDECLARATION!>object<!> {
     val x = 1
   }
 }

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

@@ -2,19 +2,19 @@
 package test
 
 class A {
-    object Companion
+    <!REDECLARATION!>object Companion<!>
 
-    companion object
+    companion <!REDECLARATION!>object<!>
 }
 
 class B {
-    companion object Named
+    companion <!REDECLARATION!>object Named<!>
 
-    object Named
+    <!REDECLARATION!>object Named<!>
 }
 
 class C {
-    class Named
+    <!REDECLARATION!>class Named<!>
 
-    companion object Named
+    companion <!REDECLARATION!>object Named<!>
 }