Fix use-after-move cases

[andrjohns]

Summary

There were two cases in the Math library which used a matrix input after std::forward-ing it into a ref or an arena type. This wasn't an issue unless rvalues were passed as inputs, at which point the ref/arena would move-construct and leave an empty matrix:

#include <stan/math.hpp>
#include <iostream>

int main() {
  Eigen::MatrixXd x(1, 1);
  x << 2.0;
  Eigen::Matrix<stan::math::var, -1, -1> y(1, 1);
  y << 0.5;
  auto r = stan::math::columns_dot_product(std::move(x), y); //segfaults
  std::cout << r(0).val() << std::endl; 
  return 0;
}

I (and claude) could only find this in the rev overloads for cholesky_decompose and columns_dot_product, but not sure if there's a good way to add a more universal test for this.

Tests

N/A - tests should still pass

Side Effects

N/A

Release notes

Fix use-after-move in cholesky_decompose and columns_dot_product

Checklist

• Copyright holder: (fill in copyright holder information)

  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

[WardBrian]

I (and claude) could only find this in the rev overloads for cholesky_decompose and columns_dot_product, but not sure if there's a good way to add a more universal test for this.

I think we could extend the test generation we use for expressions to generate calls with temporaries, maybe? Could not be worth the hassle