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/classes/delegatedGenericImplementation.fir.kt.txt

@@ -17,23 +17,23 @@ class Test1<E : Any?> : IBase<E> {
   }
 
   override fun <B : Any?> foo(a: E, b: B) {
-    <this>.#<$$delegate_0>.foo<B>(a = a, b = b)
+    <this>.#$$delegate_0.foo<B>(a = a, b = b)
   }
 
   override val <C : Any?> C.id: Map<E, C>?
     override get(): Map<E, C>? {
-      return (<this>.#<$$delegate_0>, <this>).<get-id><C>()
+      return (<this>.#$$delegate_0, <this>).<get-id><C>()
     }
 
   override var <D : Any?> List<D>.x: D?
     override get(): D? {
-      return (<this>.#<$$delegate_0>, <this>).<get-x><D>()
+      return (<this>.#$$delegate_0, <this>).<get-x><D>()
     }
     override set(<set-?>: D?) {
-      (<this>.#<$$delegate_0>, <this>).<set-x><D>(<set-?> = <set-?>)
+      (<this>.#$$delegate_0, <this>).<set-x><D>(<set-?> = <set-?>)
     }
 
-  local /* final field */ val <$$delegate_0>: IBase<E> = i
+  private /* final field */ val $$delegate_0: IBase<E> = i
 
 }