KT-5488 Invalid ambiguity between plus and plusAssign

#KT-5488  Fixed

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

@@ -0,0 +1,19 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    val c: C = C()
+}
+
+fun C.plus(a: Any): C = this
+fun C.plusAssign(a: Any) {}
+
+class C1 {
+    var c: C = C()
+}
+
+fun test() {
+    val c = C()
+    c.c += ""
+    var c1 = C1()
+    c1.c <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> ""
+}