KT-2227 Call to toString missing when interpolating string literal.

Test only.

 #KT-2227

js/js.translator/testFiles/expression/string/cases/kt2227_2.kt

@@ -0,0 +1,16 @@
+package foo
+
+var i = 0
+
+class A() {
+  fun toString():  String {
+      i++
+      return "bar"
+  }
+}
+
+fun box(): Boolean {
+    val a = A()
+    val s = "$a == $a"
+    return s == "bar == bar" && i == 2
+}