Fix NoSuchMethodError for data classes with private parameters

Introduced in a07909bb52

compiler/testData/codegen/boxWithStdlib/dataClasses/privateValParams.kt

@@ -0,0 +1,10 @@
+data class D(private val x: Long, private val y: Char)
+
+fun box(): String {
+    val d1 = D(42L, 'a')
+    val d2 = D(42L, 'a')
+    if (d1 != d2) return "Fail equals"
+    if (d1.hashCode() != d2.hashCode()) return "Fail hashCode"
+    if (d1.toString() != d2.toString()) return "Fail toString"
+    return "OK"
+}