BUG: infer_dtype_from_scalar with pandas_dtype=True doesn't infer IntervalDtype
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd; pd.__version__
Out[1]: '0.26.0.dev0+1348.g18bd98fde'

In [2]: from pandas.core.dtypes.cast import infer_dtype_from_scalar

In [3]: # this is fine 
   ...: infer_dtype_from_scalar(pd.Interval(0, 1), pandas_dtype=False)
Out[3]: (numpy.object_, Interval(0, 1, closed='right'))

In [4]: # this is should infer interval[int64] as dtype 
   ...: infer_dtype_from_scalar(pd.Interval(0, 1), pandas_dtype=True)
Out[4]: (numpy.object_, Interval(0, 1, closed='right'))
```
#### Problem description
`infer_dtype_from_scalar` with `pandas_dtype=True` infers the dtype for `Interval` objects as `np.object_` instead of as an `IntervalDtype`.

#### Expected Output
I'd expect `Out[4]` to have the dtype as `IntervalDtype(subtype=np.int64)`.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 18bd98fde954806cf0247aa961cc2e9e567871ba
python           : 3.7.3.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.26.0.dev0+1348.g18bd98fde
numpy            : 1.16.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.6.0.post20191030
Cython           : 0.29.13
pytest           : 5.3.2
hypothesis       : 4.23.6
sphinx           : 1.8.5
blosc            : None
feather          : None
xlsxwriter       : 1.1.8
lxml.etree       : 4.3.3
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : 1.2.1
fastparquet      : 0.3.0
gcsfs            : None
lxml.etree       : 4.3.3
matplotlib       : 3.1.0
numexpr          : 2.6.9
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : 0.11.1
pytables         : None
pytest           : 5.3.2
s3fs             : 0.2.1
scipy            : 1.2.1
sqlalchemy       : 1.3.4
tables           : 3.5.2
xarray           : 0.12.1
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.8

</details>
