KT-5488 Invalid ambiguity between plus and plusAssign

#KT-5488  Fixed

compiler/testData/diagnostics/tests/operatorsOverloading/plusAssignOnArray.kt

@@ -0,0 +1,20 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    fun get(i: Int): C = this
+}
+
+fun C.plus(a: Any): C = this
+fun C.plusAssign(a: Any) {}
+
+class C1 {
+    fun get(i: Int): C = C()
+    fun set(i: Int, v: C) {}
+}
+
+fun test() {
+    val c = C()
+    c[0] += ""
+    var c1 = C1()
+    c1[0] <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> ""
+}