PSI2IR: Fix delegated members generation

When generating bodies for members implemented by delegation, invoke
corresponding delegate member, not an interface member. Otherwise we
might lose platform-specific nullability information in case of mixed
Kotlin-Java hierarchies, as in
implicitNotNullOnDelegatedImplementation.kt

compiler/testData/codegen/boxAgainstJava/notNullAssertions/delegation.kt

@@ -1,15 +1,4 @@
-// IGNORE_BACKEND: JVM_IR
-// FILE: Delegation.java
-
-public class Delegation {
-    public static class ReturnNull {
-        public String foo() {
-            return null;
-        }
-    }
-}
-
-// FILE: 1.kt
+// FILE: delegation.kt
 
 interface Tr {
     fun foo(): String
@@ -19,8 +8,7 @@ class DelegateTo : Delegation.ReturnNull(), Tr {
     override fun foo() = super<Delegation.ReturnNull>.foo()
 }
 
-class DelegateFrom : Tr by DelegateTo() {
-}
+class DelegateFrom : Tr by DelegateTo()
 
 fun box(): String {
     try {
@@ -31,3 +19,13 @@ fun box(): String {
         return "OK"
     }
 }
+
+// FILE: Delegation.java
+
+public class Delegation {
+    public static class ReturnNull {
+        public String foo() {
+            return null;
+        }
+    }
+}