REGR: Barplot broken on Index(dtype='object')
**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
df = pd.DataFrame(
    {
        'a': [1, 2],
    },
    index=pd.Index([0, 'Total'])
)
df.plot.bar()
```

#### Problem description

On pandas 1.2.0, this raises:
```
Traceback (most recent call last):
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 706, in astype
    casted = self._values.astype(dtype, copy=copy)
ValueError: invalid literal for int() with base 10: 'Total'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "testplot.py", line 21, in <module>
    df.plot.bar()
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/plotting/_core.py", line 1113, in bar
    return self(kind="bar", x=x, y=y, **kwargs)
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/plotting/_core.py", line 955, in __call__
    return plot_backend.plot(data, kind=kind, **kwargs)
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/plotting/_matplotlib/__init__.py", line 61, in plot
    plot_obj.generate()
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py", line 280, in generate
    self._make_plot()
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/plotting/_matplotlib/core.py", line 1434, in _make_plot
    self.tick_pos = ax.convert_xunits(self.ax_index).astype(np.int)
  File "/home/raillard/WFH/LISA/.lisa-venv-3.8/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 708, in astype
    raise TypeError(
TypeError: Cannot cast Index to dtype int64
```

#### Expected Output

On pandas 1.1.5, it works without issues and produces a plot with 2 bars, one labeled "0" and the other one "Total" as expected.

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

<details>

Python 3.8.7 (default, Dec 24 2020, 17:53:09) 
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.8.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.10.3-arch1-1
Version          : #1 SMP PREEMPT Sun, 27 Dec 2020 10:50:46 +0000
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.2.0
numpy            : 1.19.4
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.1.1
Cython           : None
pytest           : 6.2.1
hypothesis       : None
sphinx           : 3.4.2
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
