BUG: freq set to NONE when resampling pd.Multiindex (introduced in v1.1.0)
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- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---
## Update/Summary

When dealing with a pd.multiindex the frequency `df.loc[i].index.freq` has not been set. This behavior has been introduced in v1.1.0

> ```
> idx = pd.Index(range(2), name="A")
> dti = pd.date_range("2020-01-01", periods=7, freq="D", name="B")
> mi = pd.MultiIndex.from_product([idx, dti])
> 
> df = pd.DataFrame(np.random.randn(14, 2), index=mi)
> 
> >>> df.loc[0].index
> ```
> 
> `df.loc[0].index` matches `dti`, so it would be nice to get the `freq="D"` back on that.


> using code sample in [#35563 (comment)](https://github.com/pandas-dev/pandas/issues/35563#issuecomment-670246757) points to #31315
> 
> [c988567](https://github.com/pandas-dev/pandas/commit/c9885673d6a6f9f433c570d65b664b0c2ec8c6f0) is the first bad commit
> commit [c988567](https://github.com/pandas-dev/pandas/commit/c9885673d6a6f9f433c570d65b664b0c2ec8c6f0)
> Author: jbrockmendel [jbrockmendel@gmail.com](mailto:jbrockmendel@gmail.com)
> Date: Sun Feb 9 07:01:20 2020 -0800
> 
> ```
> REF: tighten what we accept in TimedeltaIndex._simple_new (#31315)
> ```

## Original post

#### Code Sample, a copy-pastable example

```python
import numpy as np
import pandas as pd

idx_level_0 = np.repeat([1, 2], 5)
dates = np.tile(
    ["2020-01-01", "2020-01-02", "2020-01-04", "2020-01-06", "2020-01-07"], 2
)
values1 = [1, 2, 3, 4, 5]
values2 = [6, 7, 8, 9, 10]

df = pd.DataFrame(
    {"idx_level_0": idx_level_0, "dates": dates, "values": [*values1, *values2]}
)
df["dates"] = pd.to_datetime(df["dates"])
df = df.set_index(["idx_level_0", "dates"], drop=True)

df = df.groupby("idx_level_0").resample("D", level="dates").last()

# The following assertion is working properly in pandas v1.0.5
# It throws an error in pandas v1.1.0
assert df.index.get_level_values(1).freq == "D"
```

#### Problem description

When resampling a `groupby`-object, the frequency will incorrectly be set to `None`.

#### Expected Output
The frequency should be set according to the resampled frequency.

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

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.8.2.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : de_DE.cp1252
pandas           : 1.1.0
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 45.3.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>

