IR: Use name \$\$delegate_n for the field for interface delegation.

Without this change, these fields could have names such as
`<no name provided>$SuperClass$delegate` for something like
`object SuperClass by I { }`.

compiler/testData/ir/irText/classes/delegatedImplementationWithExplicitOverride.txt

@@ -53,7 +53,7 @@ FILE fqName:<root> fileName:/delegatedImplementationWithExplicitOverride.kt
       BLOCK_BODY
         DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
         INSTANCE_INITIALIZER_CALL classDescriptor='CLASS CLASS name:C modality:FINAL visibility:public superTypes:[<root>.IFooBar]'
-    FIELD DELEGATE name:C$IFooBar$delegate type:<root>.FooBarImpl visibility:private [final]
+    FIELD DELEGATE name:$$delegate_0 type:<root>.FooBarImpl visibility:private [final]
       EXPRESSION_BODY
         GET_OBJECT 'CLASS OBJECT name:FooBarImpl modality:FINAL visibility:public superTypes:[<root>.IFooBar]' type=<root>.FooBarImpl
     FUN DELEGATED_MEMBER name:foo visibility:public modality:OPEN <> ($this:<root>.C) returnType:kotlin.Unit
@@ -62,7 +62,7 @@ FILE fqName:<root> fileName:/delegatedImplementationWithExplicitOverride.kt
       $this: VALUE_PARAMETER name:<this> type:<root>.C
       BLOCK_BODY
         CALL 'public open fun foo (): kotlin.Unit declared in <root>.FooBarImpl' type=kotlin.Unit origin=null
-          $this: GET_FIELD 'FIELD DELEGATE name:C$IFooBar$delegate type:<root>.FooBarImpl visibility:private [final]' type=<root>.FooBarImpl origin=null
+          $this: GET_FIELD 'FIELD DELEGATE name:$$delegate_0 type:<root>.FooBarImpl visibility:private [final]' type=<root>.FooBarImpl origin=null
             receiver: GET_VAR '<this>: <root>.C declared in <root>.C.foo' type=<root>.C origin=null
     FUN name:bar visibility:public modality:OPEN <> ($this:<root>.C) returnType:kotlin.Unit
       overridden: