BUG: concat_{categorical, compat} leads to erroneous result on non-ns datetime-EA 
Code to reproduce:

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

np_datetimes = np.array([datetime.date(2010, 1, 1)], dtype="datetime64[D]")
other = pd.array(["a", "b"], dtype="category")
pd.core.dtypes.concat.concat_categorical([np_datetimes, other])
# outputs:
#   array([Timestamp('1970-01-01 00:00:00.000014610'), 'a', 'b'], dtype=object)
# expected either one of
#   a) array([Timestamp('2010-01-01 00:00:00'), 'a', 'b'], dtype=object)
#   b) array([datetime.date(2010, 1, 1), 'a', 'b'], dtype=object)
```

This happens as `concat_datetime` / `_convert_datetimelike_to_object` assumes that datetimes are nanoseconds only.