Fix "No error on increment or augmented assignment when 'get' is an operator but 'set' is not" #KT-11300

compiler/testData/diagnostics/tests/operatorsOverloading/kt11300.kt

@@ -0,0 +1,11 @@
+class A {
+    operator fun get(x: Int): Int = x
+    fun set(<!UNUSED_PARAMETER!>x<!>: Int, <!UNUSED_PARAMETER!>y<!>: Int) {} // no `operator` modifier
+}
+
+fun main(args: Array<String>) {
+    val a = A()
+    <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!>++
+    <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!> += 3
+    <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!> = a[1] + 3
+}