Fix usages of fabs in check_symmetric with llvm17

[WardBrian]

Summary

This fixes an issue found in llvm17 (really libc++ 17, clang version seems to not be the important factor) which broke argument-dependent-lookup of the fabs function.
So far as I can tell, the only place we were depending on this that could actually get std::complex inputs is in check_symmetric.

To be completely honest, I'm not sure why this specific change fixes the issue, or if there is something better.

Reminder: we probably should not be using std::complex in Stan Math!

Tests

I added some tests, but because this is in a bleeding edge compiler they won't make much of a difference immediately. They will be run in the next bleeding edge pipeline run.

Side Effects

I don't believe so

Release notes

Fixed a compilation issue with the check_symmetric function on complex-valued inputs under LLVM 17

Checklist

• Copyright holder: Simons Foundation

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[andrjohns]

LGTM