Generate 'toString()': Permit for data classes

#KT-14122 Fixed

idea/testData/codeInsight/generate/toString/singleTemplate/dataClass.kt.after

@@ -0,0 +1,6 @@
+// GENERATOR: SINGLE_TEMPLATE
+data class A(val n: Int, val s: String?) {
+    override fun toString(): String {
+        return "A(n=$n, s=$s)"
+    }
+}