Fix exception from data class codegen for light classes

EA-66827

compiler/testData/diagnostics/testsWithStdLib/regression/ea66827_dataClassWrongToString.kt

@@ -0,0 +1,7 @@
+data class A(val x: Int) {
+    fun toArray(): IntArray =
+            intArrayOf(x)
+
+    override fun <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>toString<!>() =
+            toArray().takeWhile { it != -1 } // .joinToString()
+}

compiler/testData/diagnostics/testsWithStdLib/regression/ea66827_dataClassWrongToString.txt

@@ -0,0 +1,12 @@
+package
+
+kotlin.data() public final class A {
+    public constructor A(/*0*/ x: kotlin.Int)
+    public final val x: kotlin.Int
+    public final /*synthesized*/ fun component1(): kotlin.Int
+    public final /*synthesized*/ fun copy(/*0*/ x: kotlin.Int = ...): A
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public final fun toArray(): kotlin.IntArray
+    public open override /*1*/ fun toString(): kotlin.List<kotlin.Int>
+}