fix(forms): ensure select[multiple] retains selections

If you bound an array to select[multiple] via ngModel and subsequently
changed the options to select from, the UI would drop any selections
made since by the user. This was due to
SelectMultipleControlValueAccessor not keeping a reference to the new
model arrays it generated when users interacted with the select control.
Update code to keep the reference.

Closes #12527
Closes #12654

modules/@angular/forms/src/directives/select_multiple_control_value_accessor.ts

@@ -99,6 +99,7 @@ export class SelectMultipleControlValueAccessor implements ControlValueAccessor
           }
         }
       }
+      this.value = selected;
       fn(selected);
     };
   }