API: Index.__getitem__(True) returns 2D ndarray
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

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


### Reproducible Example

```python
Not clear if this is intentional:


import numpy as np
import pandas as pd

arr = np.random.randn(4)
idx = pd.Index(arr)

idx[True]

>>> idx[True]
array([[ 1.39365849, -0.03655461,  0.17112114, -1.4612337 ]])
```



### Issue Description

For other cases where indexing on index._data returns 2D (e.g. `idx[:, None]`) we issue a FutureWarning, but this goes through a different code path.

### Expected Behavior

I don't have an opinion here, just think whatever we do should be intentional

### Installed Versions

<details>

Replace this line with the output of pd.show_versions()

</details>
