ENH: Return DatetimeIndex or TimedeltaIndex bins for q/cut when input is datelike
- [x] closes #19891
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=h1) Report
> Merging [#20956](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7cd2679982138e1e7f9f1a759827647e6e521d43?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20956/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20956      +/-   ##
==========================================
+ Coverage    91.9%   91.91%   +<.01%     
==========================================
  Files         158      158              
  Lines       49690    49695       +5     
==========================================
+ Hits        45670    45677       +7     
+ Misses       4020     4018       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <100%> (ø)` | :arrow_up: |
| #single | `41.95% <20%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/20956/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `94.73% <100%> (+1.36%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=footer). Last update [7cd2679...17e96c4](https://codecov.io/gh/pandas-dev/pandas/pull/20956?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Simplified this a bit with the Index constructor; however, I wanted to avoid turing all bins into an Index type unless this is an API change we want to make. 

Also, I created #20964 since the Index constructor does not correctly localize values with a datetimetz dtype specified.  Updated and ready for a final review @jreback 