BUG: Preserve data order when stacking unsorted levels (#16323)
This should fix the df.stack() problems in #16323-- the levels being used to build the new index were those of the original frame, which fails when the levels get sorted.

 - [X] closes #16323
 - [X] tests added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=h1) Report
> Merging [#16325](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0607e03f71b52e131947d73880380796f51d72fb?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16325/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16325      +/-   ##
==========================================
- Coverage   90.37%   90.35%   -0.02%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45966    45957       -9     
- Misses       4897     4906       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <100%> (ø)` | :arrow_up: |
| #single | `40.33% <0%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.28% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=footer). Last update [0607e03...bd0eda2](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=h1) Report
> Merging [#16325](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0607e03f71b52e131947d73880380796f51d72fb?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16325/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16325      +/-   ##
==========================================
- Coverage   90.37%   90.35%   -0.03%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45966    45955      -11     
- Misses       4897     4908      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.33% <0%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.28% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.23% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=footer). Last update [0607e03...bd0eda2](https://codecov.io/gh/pandas-dev/pandas/pull/16325?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 yep seems reasonable. this was an untested case.