Generate references to overridden declarations as a separate pass

NB currently it relies on overriddenDescriptors in a FunctionDescriptor,
thus maybe incorrect for transformed declarations.

compiler/testData/ir/irText/expressions/objectClassReference.txt

@@ -6,11 +6,17 @@ FILE fqName:<root> fileName:/objectClassReference.kt
         DELEGATING_CONSTRUCTOR_CALL 'constructor Any()'
         INSTANCE_INITIALIZER_CALL classDescriptor='A'
     FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:Boolean flags:
+      overridden:
+        FUN IR_EXTERNAL_DECLARATION_STUB name:equals visibility:public modality:OPEN <> (other:kotlin.Any?) returnType:Boolean flags:
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any flags:
       VALUE_PARAMETER name:other index:0 type:kotlin.Any? flags:
     FUN FAKE_OVERRIDE name:hashCode visibility:public modality:OPEN <> ($this:kotlin.Any) returnType:Int flags:
+      overridden:
+        FUN IR_EXTERNAL_DECLARATION_STUB name:hashCode visibility:public modality:OPEN <> () returnType:Int flags:
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any flags:
     FUN FAKE_OVERRIDE name:toString visibility:public modality:OPEN <> ($this:kotlin.Any) returnType:String flags:
+      overridden:
+        FUN IR_EXTERNAL_DECLARATION_STUB name:toString visibility:public modality:OPEN <> () returnType:String flags:
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any flags:
   FUN name:test visibility:public modality:FINAL <> () returnType:Unit flags:
     BLOCK_BODY