CLN: Datetimelike._can_hold_na
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] no whatsnew, as it is internal fix

datetimelike `Index._can_hold_na` is all `False` even though these can store `NaT`.

```
idx = pd.DatetimeIndex(['2011-01-01', 'NaT'])
idx.hasnans
# True

idx._can_hold_na
# False

idx._nan_idxs
# array([], dtype=int64)
```

Because of `test_base.py` condition below, datetime-like Indexes had skipped some tests until now.
- https://github.com/pydata/pandas/blob/master/pandas/tests/test_base.py#L219

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13983?src=pr) is 85.28% (diff: 100%)

> Merging [#13983](https://codecov.io/gh/pydata/pandas/pull/13983?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13983   diff @@
==========================================
  Files           139        139          
  Lines         50543      50547     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43101      43107     +6   
+ Misses         7442       7440     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8fdfa51...5435247](https://codecov.io/gh/pydata/pandas/compare/8fdfa518105cde9ae16449578b3d75c13c8c19a5...543524700440ff9ed7e17c6d93fc5937499db6e5?src=pr)
 Updated not to rely on #13979. `_get_unique_index` can be removed after #13984.
 @jreback @jorisvandenbossche can u check? This is needed to make some `Index` dtype related fixes.
 This is looking good to me. @jreback OK?
 needs #13979 first
 @sinhrks updated to not rely on that one
 @jreback Now this PR doesn't rely on #13979. 
 @sinhrks This needs a rebase now I merged the other
 lgtm. ping on green.
 fixed lint error, and now green.
 Thanks!
