call overloaded *Assign methods for augmented assignment

idea/testData/codegen/classes/overloadPlusAssign.jet

@@ -0,0 +1,26 @@
+import java.util.*
+
+class ArrayWrapper<T>() {
+    val contents = ArrayList<T>()
+
+    fun add(item: T) {
+        contents.add(item)
+    }
+
+    fun plusAssign(rhs: ArrayWrapper<T>) {
+        contents.addAll(rhs.contents)
+    }
+
+    fun get(index: Int): T {
+        return contents.get(index)
+    }
+}
+
+fun box(): String {
+    val v1 = ArrayWrapper<String>()
+    val v2 = ArrayWrapper<String>()
+    v1.add("foo")
+    v2.add("bar")
+    v1 += v2
+    return if (v1.contents.size() == 2) "OK" else "fail"
+}

idea/testData/codegen/classes/overloadPlusAssignReturn.jet

@@ -0,0 +1,30 @@
+import java.util.*
+
+class ArrayWrapper<T>() {
+    val contents = ArrayList<T>()
+
+    fun add(item: T) {
+        contents.add(item)
+    }
+
+    fun plusAssign(rhs: ArrayWrapper<T>): ArrayWrapper<T> {
+        val result = ArrayWrapper<T>()
+        result.contents.addAll(contents)
+        result.contents.addAll(rhs.contents)
+        return result
+    }
+
+    fun get(index: Int): T {
+        return contents.get(index)
+    }
+}
+
+fun box(): String {
+    val v1 = ArrayWrapper<String>()
+    val v2 = ArrayWrapper<String>()
+    v1.add("foo")
+    val v3 = v1
+    v2.add("bar")
+    v1 += v2
+    return if (v1.contents.size() == 2 && v3.contents.size() == 1) "OK" else "fail"
+}