Fix intrinsic for String.plus for explicit calls

#KT-14567 Fixed

compiler/testData/codegen/box/operatorConventions/plusExplicit.kt

@@ -0,0 +1,29 @@
+// WITH_RUNTIME
+
+fun bar1(): String {
+    val l: List<String> = listOf("O")
+    val s = l[0].plus("K")
+    return s
+}
+
+fun bar2(): String {
+    val l: List<String?> = listOf("O")
+    val s = l[0]?.plus("K")
+    return s!!
+}
+
+fun bar3(): String {
+    val l: List<String> = listOf("O")
+    with(l[0]) {
+        return plus("K")
+    }
+    return "fail"
+}
+
+fun box(): String {
+    if (bar1() != "OK") return "fail 1"
+    if (bar2() != "OK") return "fail 2"
+    if (bar3() != "OK") return "fail 3"
+
+    return "OK"
+}