fix(forms): export AbstractFormGroupDirective

Because
- `Form` is **exported** -- see line 30/31 of
`modules/@angular/forms/src/forms.ts`: i.e., <br>`export {Form} from
'./directives/form_interface'`; and
- Methods of `Form`, which are public, have an
`AbstractFormGroupDirective` parameter;
e.g.,<br>`Form.getFormGroup(dir: AbstractFormGroupDirective):
FormGroup`.

Then it makes sense for `AbstractFormGroupDirective` to be
public/exported too. In any case, if it isn't exported then the **API
docs for `Form` don't get generated properly.**

tools/public_api_guard/forms/index.d.ts

@@ -75,6 +75,17 @@ export declare abstract class AbstractControlDirective {
     reset(value?: any): void;
 }
 
+/** @experimental */
+export declare class AbstractFormGroupDirective extends ControlContainer implements OnInit, OnDestroy {
+    asyncValidator: AsyncValidatorFn;
+    control: FormGroup;
+    formDirective: Form;
+    path: string[];
+    validator: ValidatorFn;
+    ngOnDestroy(): void;
+    ngOnInit(): void;
+}
+
 /** @experimental */
 export interface AsyncValidatorFn {
     (c: AbstractControl): any;