Add aria-controls to combobox (#5942)

src/js/select2/selection/single.js

@@ -37,6 +37,7 @@ define([
       .attr('role', 'textbox')
       .attr('aria-readonly', 'true');
     this.$selection.attr('aria-labelledby', id);
+    this.$selection.attr('aria-controls', id);
 
     this.$selection.on('mousedown', function (evt) {
       // Only respond to left clicks