Test more with GCC 16

[sean-mcmanus]

Two user reported bugs with g++-16: #14453 and #14459

We probably should do (should have done) more pro-active testing with that earlier (since we've hit issues like this before when new g++ compilers were released),

[sean-mcmanus]

Actually, I did test g++-16 and the system headers back in March (and we fixed one crash with it, #14309), but it looks like the test cases generated by Copilot were too simple to catch the other invalid IntelliSense error bugs.