title: How to get FormControl instance from ControlValueAccessor
body: <p>I've the following component:</p>
  
  <pre><code>@Component({
      selector: 'pc-radio-button',
      templateUrl: './radio-button.component.html',
      providers: [
          {provide: NG_VALUE_ACCESSOR, useExisting: forwardRef(() =&gt; FieldRadioButtonComponent), multi: true}
      ]
  })
  export class RadioButtonComponent implements ControlValueAccessor {
      ...
  }
  </code></pre>
  
  <p>I can assign and alter the value through these inputs:</p>
  
  <pre><code>&lt;pc-radio-button [formControl]="formControl"&gt;&lt;/pc-radio-button&gt;
  &lt;pc-radio-button [formControlName]="inputControlName"&gt;&lt;/pc-radio-button&gt;
  </code></pre>
  
  <p>However I need the component to have the direct access to the assigned formControl, as I need to add styles depending on its status.</p>
  
  <p>By creating an <strong>@Input() formControl</strong> does not solve the problem. As it does not cover the case when form control is assigned via formControlName.</p>