Fix for KT-4301: Too many StringBuilders on string concatenations

#KT-4301 Fixed

compiler/testData/codegen/box/strings/stringBuilderAppend.kt

@@ -0,0 +1,16 @@
+class A() {
+
+    override fun toString(): String {
+        return "A"
+    }
+}
+
+fun box() : String {
+
+    val s = "1" + "2" + 3 + 4L + 5.0 + 6F + '7' + A()
+
+    if (s != "12345.06.07A") return "fail $s"
+
+    return "OK"
+}
+