FIR: Fix interface delegation ABI

- Unifies the name of the `$$delegate_<n>` field between K1 and K2.
- Make the `$$delegate_<n>` field private

compiler/testData/ir/irText/declarations/annotations/inheritingDeprecation.fir.kt.txt

@@ -23,16 +23,16 @@ class Delegated : IFoo {
   @Deprecated(message = "")
   override val prop: String
     override get(): String {
-      return <this>.#<$$delegate_0>.<get-prop>()
+      return <this>.#$$delegate_0.<get-prop>()
     }
 
   @Deprecated(message = "")
   override val String.extProp: String
     override get(): String {
-      return (<this>.#<$$delegate_0>, <this>).<get-extProp>()
+      return (<this>.#$$delegate_0, <this>).<get-extProp>()
     }
 
-  local /* final field */ val <$$delegate_0>: IFoo = foo
+  private /* final field */ val $$delegate_0: IFoo = foo
 
 }