ENH: Implement IntervalIndex.is_overlapping
- [X] closes #23309
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This is needed in the `get_indexer` implementation for the new `IntervalIndex` behavior, as an overlapping `IntervalIndex` may return non-unique indices for a given query; seems cleaner to implement separately. Also makes sense as a general attribute for an `IntervalIndex` to have.
Hello @jschendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/interval.py`](https://github.com/jschendel/pandas/blob/f73f4c141efa5aff802a60d913b563700463fd11/pandas/core/arrays/interval.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/interval.py`](https://github.com/jschendel/pandas/blob/f73f4c141efa5aff802a60d913b563700463fd11/pandas/core/indexes/interval.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/interval/test_interval.py`](https://github.com/jschendel/pandas/blob/f73f4c141efa5aff802a60d913b563700463fd11/pandas/tests/indexes/interval/test_interval.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/interval/test_interval_tree.py`](https://github.com/jschendel/pandas/blob/f73f4c141efa5aff802a60d913b563700463fd11/pandas/tests/indexes/interval/test_interval_tree.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23327?src=pr&el=h1) Report
> Merging [#23327](https://codecov.io/gh/pandas-dev/pandas/pull/23327?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/90961f2acad5c2db6aa957e2a79df8668f5b02d1?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/23327/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23327?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23327      +/-   ##
==========================================
+ Coverage   92.31%   92.31%   +<.01%     
==========================================
  Files         161      161              
  Lines       51483    51487       +4     
==========================================
+ Hits        47525    47529       +4     
  Misses       3958     3958
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <100%> (ø)` | :arrow_up: |
| #single | `42.43% <25%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23327?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/23327/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVydmFsLnB5) | `93.02% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/23327/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `94.73% <100%> (+0.03%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23327?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/23327?src=pr&el=footer). Last update [90961f2...dd63492](https://codecov.io/gh/pandas-dev/pandas/pull/23327?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ping @jreback : this is passing now that #23353 has been merged can you merge master @jschendel can you rebase @jreback : rebased and switched to an array based implementation (see https://github.com/pandas-dev/pandas/pull/23327#discussion_r236506787) ping @jreback  thanks @jschendel nice! Thanks for the quick response to the ping @jreback!  Sorry for the delay with this PR; got a bit sidetracked with the 32bit issues.