 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)

