IntervalIndex does not accept CategoricalIndex (of interval dtype)
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.qcut(range(100), 10).value_counts().index.categories.dtype
Out[2]: interval[float64]

In [3]: pd.IntervalIndex(pd.qcut(range(100), 10).value_counts().index)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-32e3bb1a4f49> in <module>()
----> 1 pd.IntervalIndex(pd.qcut(range(100), 10).value_counts().index)

~/nobackup/repo/pandas/pandas/core/indexes/interval.py in __new__(cls, data, closed, dtype, copy, name, fastpath, verify_integrity)
    238 
    239             data = maybe_convert_platform_interval(data)
--> 240             left, right, infer_closed = intervals_to_interval_bounds(data)
    241 
    242             if (com._all_not_none(closed, infer_closed) and

TypeError: Argument 'intervals' has incorrect type (expected numpy.ndarray, got CategoricalIndex)

```

#### Problem description

From a practical point of view, the above is just handy. From a conceptual point of view, the fact that a ``Series`` or ``Index`` has ``categorical`` dtype should be irrelevant for all operations which do not directly concern categories, so again, the above should work the same as if it was not a ``CategoricalIndex``.

I think the following also reflects the same problem:
```python
In [5]: pd.IntervalIndex(pd.qcut(range(100), 10))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-3744b32bfad8> in <module>()
----> 1 pd.IntervalIndex(pd.qcut(range(100), 10))

~/nobackup/repo/pandas/pandas/core/indexes/interval.py in __new__(cls, data, closed, dtype, copy, name, fastpath, verify_integrity)
    238 
    239             data = maybe_convert_platform_interval(data)
--> 240             left, right, infer_closed = intervals_to_interval_bounds(data)
    241 
    242             if (com._all_not_none(closed, infer_closed) and

TypeError: Argument 'intervals' has incorrect type (expected numpy.ndarray, got Categorical)
```

... and hence, also passing ``index.values`` raises an error.

#### Expected Output

An ``IntervalIndex`` with the same content of the ``CategoricalIndex``.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+25.gcd0447102
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.25.2
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1153+gff6786446
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
