0.24.0rc1: infer_dtype(DatetimeIndex) returns "datetime64" not "datetime64[ns]"
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

datetime_index = pd.DatetimeIndex(['2017-08-01', '2017-08-02'])
print(datetime_index)
# DatetimeIndex(['2017-08-01', '2017-08-02'], dtype='datetime64[ns]', freq=None)
print(datetime_index.dtype)
# datetime64[ns]
infered_dtype = pd.api.types.infer_dtype(datetime_index, skipna=True)
print(infered_dtype)
# datetime64
print(pd.Index(['2017-08-01', '2017-08-02'], dtype=infered_dtype))
# Traceback (most recent call last):
#   File "/tmp/a.py", line 8, in <module>
#     print(pd.Index(['2017-08-01', '2017-08-02'], dtype=infered_dtype))
#   File "/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/base.py", line 308, in __new__
#     dtype=dtype, **kwargs)
#   File "/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/datetimes.py", line 303, in __new__
#     int_as_wall_time=True)
#   File "/usr/local/lib/python3.7/dist-packages/pandas/core/arrays/datetimes.py", line 368, in _from_sequence
#     ambiguous=ambiguous, int_as_wall_time=int_as_wall_time)
#   File "/usr/local/lib/python3.7/dist-packages/pandas/core/arrays/datetimes.py", line 1706, in sequence_to_dt64ns
#     dtype = _validate_dt64_dtype(dtype)
#   File "/usr/local/lib/python3.7/dist-packages/pandas/core/arrays/datetimes.py", line 1993, in _validate_dt64_dtype
#     .format(dtype=dtype))
# ValueError: Unexpected value for 'dtype': 'datetime64'. Must be 'datetime64[ns]' or DatetimeTZDtype'.
```

#### Problem description

We can't use dtype inferred from `DatetimeIndex` to convert an `Index` to `DatetimeIndex` with pandas 0.24.0rc1.

[pyarrow](https://github.com/apache/arrow/tree/master/python) uses this logic to convert Arrow objects to pandas objects.

FYI: Here are related codes:

  * https://github.com/apache/arrow/blob/master/python/pyarrow/pandas_compat.py#L36
  * https://github.com/apache/arrow/blob/master/python/pyarrow/pandas_compat.py#L114
  * https://github.com/apache/arrow/blob/master/python/pyarrow/pandas_compat.py#L235
  * https://github.com/apache/arrow/blob/master/python/pyarrow/pandas_compat.py#L789

FYI: Here is an error in pyarrow test: https://travis-ci.org/kszucs/crossbow/builds/478558634#L2724-L2788

#24478 introduces a validation for `datetime64`. But `pd.api.types.infer_dtype` still returns `'datetime64'` for `DatetimeIndex`.

The following change fixes this problem. But I'm not sure whether this is a regression or pyarrow's use case is wrong.

```diff
diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index 85eb6c342..6271d1204 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -928,7 +928,7 @@ _TYPE_MAP = {
     'U': 'unicode' if PY2 else 'string',
     'bool': 'boolean',
     'b': 'boolean',
-    'datetime64[ns]': 'datetime64',
+    'datetime64[ns]': 'datetime64[ns]',
     'M': 'datetime64',
     'timedelta64[ns]': 'timedelta64',
     'm': 'timedelta64',
```

#### Expected Output

```python
import pandas as pd

datetime_index = pd.DatetimeIndex(['2017-08-01', '2017-08-02'])
print(datetime_index)
# DatetimeIndex(['2017-08-01', '2017-08-02'], dtype='datetime64[ns]', freq=None)
print(datetime_index.dtype)
# datetime64[ns]
infered_dtype = pd.api.types.infer_dtype(datetime_index, skipna=True)
print(infered_dtype)
# datetime64[ns]
print(pd.Index(['2017-08-01', '2017-08-02'], dtype=infered_dtype))
# DatetimeIndex(['2017-08-01', '2017-08-02'], dtype='datetime64[ns]', freq=None)
```

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

<details>

```text
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.0-1-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: ja_JP.UTF-8
LOCALE: ja_JP.UTF-8

pandas: 0.24.0rc1
pytest: 3.10.1
pip: 18.1
setuptools: 40.6.2
Cython: 0.29.2
numpy: 1.16.0rc2
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
```

</details>
