fix(dialog): don't assign aria-label to close button if button has text (#11093)

No longer adds the `aria-label` to the `mat-dialog-close` directive, if the element that it's being applied to has text. This prevents the label from being read instead of the button text.

Fixes #11084.

tools/public_api_guard/lib/dialog.d.ts

@@ -45,6 +45,7 @@ export declare const matDialogAnimations: {
 };
 
 export declare class MatDialogClose implements OnInit, OnChanges {
+    _hasAriaLabel?: boolean;
     _matDialogClose: any;
     ariaLabel: string;
     dialogRef: MatDialogRef<any>;