Filter identifier doesn't work for grouped taxonomy filter

Proposed patch with solution.

Thanks for reporting but MR should be against 11.x as current development branch

Also will need test coverage showing this issue.

I'm sorry, but I can't reproduce the issue with the information that is given. Here are the steps I took in an attempt to follow the steps given in the issue summary:

1. Created a new view. I chose to display taxonomy terms.
2. Added a new exposed Term ID filter.
3. Added the filter to a new filter group.
4. Saved the view.
5. Visited the view and verified that the Term ID filter works.
6. Edited the view and changed the Term ID filter's identifier.
7. Saved the view again.
8. Visited the view and verified that the Term ID filter works.

If you can tell me where I went wrong, then I'll be happy to try again.