Fix #13306: Hour overflow in tz-aware datetime conversions.
- [x] closes #13306
- [x] tests passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
      Bug: tz-converting tz-aware DateTimeIndex relied on index being sorted for correct results.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13313?src=pr) is **84.22%**

> Merging [#13313](https://codecov.io/gh/pydata/pandas/pull/13313?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13313   diff @@
==========================================
  Files           138        138          
  Lines         50713      50666    -47   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42715      42672    -43   
+ Misses         7998       7994     -4   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [fcd73ad...4ca6f0c](https://codecov.io/gh/pydata/pandas/compare/fcd73ad2e7482414b61d47056c6c9c220b11702c...4ca6f0cdff56f9ba0a6ec222a93825ad0d2e572c)
 how would you know this worked without tests?

this needs 2 paths 1 if is_montonic (iow is sorted); 1 if not
 let me know if you need anything else from me
 pls add a whatsnew note as well (Bug fix section)
 ok, thanks. i'll do that stuff tonight
 sure, ping when ready
 can you add a whatsnew entry, otherwise lgtm.
 Figured out how to add whatsnew entry :)
 sigh, will have to deal with conflict tonight. cannot do it at work. it's in the v0.18.2.txt file. feel free to resolve it yourself if you don't want to wait until tonight
 i'm done. take over if you're ok with it
 :+1:
 thanks @uwedeportivo 

yeah ideally we would check sortedness in the cython code (e.g. `is_monotonic` works for this and is fast. Though I can't produce a counterexample.
