BUG: Fix tz-aware DatetimeIndex +/- TimedeltaIndex/timedelta64 array
- [X] closes #17558
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Added the whatsnew entry under 0.22.0, but could possibly go under 0.21.1.  This PR is a prereq for #18558.