FIR: Fix interface delegation ABI

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

compiler/fir/analysis-tests/testData/resolve/delegatedSuperType.fir.txt

@@ -17,7 +17,7 @@ FILE: delegatedSuperType.kt
             super<R|kotlin/Any|>()
         }
 
-        local final field <$$delegate_0>: R|A| = R|<local>/b|
+        private final field $$delegate_0: R|A| = R|<local>/b|
 
         public final val b: R|B| = R|<local>/b|
             public get(): R|B|

compiler/fir/analysis-tests/testData/resolve/diagnostics/anonymousObjectByDelegate.fir.txt

@@ -13,7 +13,7 @@ FILE: anonymousObjectByDelegate.kt
                 super<R|kotlin/Any|>()
             }
 
-            local final field <$$delegate_0>: R|B| = this@R|/test_1|.R|/A.b|
+            private final field $$delegate_0: R|B| = this@R|/test_1|.R|/A.b|
 
         }
 
@@ -24,7 +24,7 @@ FILE: anonymousObjectByDelegate.kt
                 super<R|kotlin/Any|>()
             }
 
-            local final field <$$delegate_0>: R|B| = this@R|/test_2|.R|/A.b|
+            private final field $$delegate_0: R|B| = this@R|/test_2|.R|/A.b|
 
         }
 

compiler/fir/analysis-tests/testData/resolve/diagnostics/delegationInInterface.fir.txt

@@ -6,7 +6,7 @@ FILE: delegationInInterface.kt
 
     }
     public abstract interface B : R|A| {
-        local final field <$$delegate_0>: R|A| = <Unresolved name: a>#
+        private final field $$delegate_0: R|A| = <Unresolved name: a>#
 
         public abstract val a: R|A|
             public get(): R|A|
@@ -15,6 +15,6 @@ FILE: delegationInInterface.kt
     public final val test: R|A| = R|/A.A|()
         public get(): R|A|
     public abstract interface C : R|A| {
-        local final field <$$delegate_0>: R|A| = R|/test|
+        private final field $$delegate_0: R|A| = R|/test|
 
     }

compiler/fir/analysis-tests/testData/resolve/diagnostics/superCallWithDelegation.fir.txt

@@ -8,7 +8,7 @@ FILE: superCallWithDelegation.kt
             super<R|kotlin/Any|>()
         }
 
-        local final field <$$delegate_0>: R|A| = R|<local>/a|
+        private final field $$delegate_0: R|A| = R|<local>/a|
 
         private final val a: R|A| = R|<local>/a|
             private get(): R|A|

compiler/fir/analysis-tests/testData/resolve/inlineClasses/inlineClassDeclaration.fir.txt

@@ -103,7 +103,7 @@ FILE: inlineClassDeclaration.kt
             super<R|kotlin/Any|>()
         }
 
-        local final field <$$delegate_0>: R|kotlin/Comparable<kotlin/Int>| = R|<local>/x|
+        private final field $$delegate_0: R|kotlin/Comparable<kotlin/Int>| = R|<local>/x|
 
         public final val x: R|kotlin/Int| = R|<local>/x|
             public get(): R|kotlin/Int|

compiler/fir/analysis-tests/testData/resolve/smartcasts/smartcastInByClause.dot

@@ -121,7 +121,7 @@ digraph smartcastInByClause_kt {
         super<R|kotlin/Any|>()
     }
 
-    local final field <$$delegate_0>: R|Base| = R|/Derived.Derived|(R|<local>/a|.R|/A.index|)
+    private final field $$delegate_0: R|Base| = R|/Derived.Derived|(R|<local>/a|.R|/A.index|)
 
     public final val x: R|kotlin/Int| = R|<local>/a|.R|/A.index|
         public get(): R|kotlin/Int|

compiler/fir/analysis-tests/testData/resolve/smartcasts/smartcastInByClause.fir.txt

@@ -30,7 +30,7 @@ FILE: smartcastInByClause.kt
                 super<R|kotlin/Any|>()
             }
 
-            local final field <$$delegate_0>: R|Base| = R|/Derived.Derived|(R|<local>/a|.R|/A.index|)
+            private final field $$delegate_0: R|Base| = R|/Derived.Derived|(R|<local>/a|.R|/A.index|)
 
             public final val x: R|kotlin/Int| = R|<local>/a|.R|/A.index|
                 public get(): R|kotlin/Int|