 BUG: fix reindexing MultiIndex with categorical datetime-like level 
closes #21390

This fixes the bug, but is not really a general solution. However, I would like to keep that for another PR (won't have time for this before 0.23.2), also because the underlying reason is more widely present than in just the MultiIndex.values (will open a separate issue about this -> https://github.com/pandas-dev/pandas/issues/21658).

I added some tests of the bug at several levels where it surfaces (groupby, reindex, index.get_indexer)


Hello @jorisvandenbossche! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on June 29, 2018 at 09:13 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21657?src=pr&el=h1) Report
> Merging [#21657](https://codecov.io/gh/pandas-dev/pandas/pull/21657?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0b63e81dfcd7860a8c30e0837238ff9cc7bbe88d?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21657/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21657      +/-   ##
==========================================
- Coverage    91.9%    91.9%   -0.01%     
==========================================
  Files         154      154              
  Lines       49656    49559      -97     
==========================================
- Hits        45637    45546      -91     
+ Misses       4019     4013       -6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.03% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `94.9% <100%> (-0.06%)` | :arrow_down: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `83.95% <0%> (-3.65%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.41% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.9% <0%> (-0.05%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.19% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.83% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.63% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.78% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.67% <0%> (ø)` | :arrow_up: |
| ... and [7 more](https://codecov.io/gh/pandas-dev/pandas/pull/21657/diff?src=pr&el=tree-more) | |

------

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