add describedByIds to input

Author: adolgachev

src/material/input/input.ts

@@ -550,6 +550,17 @@ export class MatInput
     }
   }
 
+  /**
+   * Implemented as part of MatFormFieldControl.
+   * @docs-private
+   */
+  get describedByIds(): string[] {
+    const element = this._elementRef.nativeElement;
+    const existingDescribedBy = element.getAttribute('aria-describedby');
+
+    return existingDescribedBy?.split(' ') || [];
+  }
+
   /**
    * Implemented as part of MatFormFieldControl.
    * @docs-private