fix(autocomplete): set aria-haspopup (#15079)

Based on the [autocomplete example from the a11y guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/examples/combobox/aria1.0pattern/combobox-autocomplete-both.html), adds `aria-haspopup` to the autocomplete trigger input.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -105,6 +105,7 @@ export function getMatAutocompleteMissingPanelError(): Error {
     '[attr.aria-activedescendant]': '(panelOpen && activeOption) ? activeOption.id : null',
     '[attr.aria-expanded]': 'autocompleteDisabled ? null : panelOpen.toString()',
     '[attr.aria-owns]': '(autocompleteDisabled || !panelOpen) ? null : autocomplete?.id',
+    '[attr.aria-haspopup]': '!autocompleteDisabled',
     // Note: we use `focusin`, as opposed to `focus`, in order to open the panel
     // a little earlier. This avoids issues where IE delays the focusing of the input.
     '(focusin)': '_handleFocus()',

src/lib/autocomplete/autocomplete.spec.ts

@@ -482,6 +482,15 @@ describe('MatAutocomplete', () => {
       expect(fixture.componentInstance.stateCtrl.value).toBe('hello');
     });
 
+    it('should set aria-haspopup depending on whether the autocomplete is disabled', () => {
+      expect(input.getAttribute('aria-haspopup')).toBe('true');
+
+      fixture.componentInstance.autocompleteDisabled = true;
+      fixture.detectChanges();
+
+      expect(input.getAttribute('aria-haspopup')).toBe('false');
+    });
+
   });
 
   it('should have the correct text direction in RTL', () => {