CI Add Clang-18 C++23 regression tests

[jarzec]

As stated in the tile. After #1088 cppfront compiles using Clang-18 with C++23 selected. For this to work an up-to-date libc++ has to be used.

• The build-cppfornt.yml workflow file is modified as Clang-18 accepts c++23 as language standard option.
• The regression-tests.yml is modified to include the new compiler options.
• Reference files are added.
• run-tests.sh is modified to make the label option (-l) mandatory. If it is not provided error reporting fails.

The added compiler option is the only one that allows for all tests to compile on Ubuntu. Out of all the other options only mac-14 | clang++ | c++2b | default and win-22 | cl.exe | c++latest | default correctly lower and compile all the regression test.

[hsutter]

Thanks!