Ensure that DatetimeArray keeps reference to original data
(cherry picked from commit 49f6495ba5ceef35a7a962f20e8b863c544592b7)

This avoids a segfault I (and possibly @jbrockmendel) were seeing on our datetimearray branches from us freeing a region of memory we didn't allocate.

I haven't really groked things yet, but `_libs/reduction.pyx::Slider` is doing some strange things with the data of the array passed to it (overwriting the buffer's `.data` and `.strides` attributes). One of the arguments, `buff`, is a slice on the original index. It *seems* like ensuring that the ndarray backing the DatetimeArray sliced index is the same as the original index avoids the segfault.
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_datetimelike.py`](https://github.com/TomAugspurger/pandas/blob/197e9db309925038547c62d61dc87180bac1bb91/pandas/tests/arrays/test_datetimelike.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23956?src=pr&el=h1) Report
> Merging [#23956](https://codecov.io/gh/pandas-dev/pandas/pull/23956?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/db8d33e26395eb567a43b121258b0a05ddc33706?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23956/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23956?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23956   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         161      161           
  Lines       51471    51471           
=======================================
  Hits        47515    47515           
  Misses       3956     3956
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23956?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/23956?src=pr&el=footer). Last update [db8d33e...a77a24a](https://codecov.io/gh/pandas-dev/pandas/pull/23956?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. thanks!