Fix complex matrix division miscompilation

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

Fixed an issue where operator/ was not generating the correct C++ for complex linear algebra types.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.83%. Comparing base (9e9953f) to head (5b4e80b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1421 +/- ## ========================================== + Coverage 89.81% 89.83% +0.01% ========================================== Files 63 63 Lines 10480 10484 +4 ========================================== + Hits 9413 9418 +5 + Misses 1067 1066 -1 ``` | [Files](https://app.codecov.io/gh/stan-dev/stanc3/pull/1421?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev) | Coverage Δ | | |---|---|---| | [src/stan\_math\_backend/Lower\_expr.ml](https://app.codecov.io/gh/stan-dev/stanc3/pull/1421?src=pr&el=tree&filepath=src%2Fstan_math_backend%2FLower_expr.ml&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stan-dev#diff-c3JjL3N0YW5fbWF0aF9iYWNrZW5kL0xvd2VyX2V4cHIubWw=) | `93.48% <100.00%> (+0.33%)` | :arrow_up: |