Add new compiler type for Flang

[partouf]

Feb 05 08:37:01 librarybuilder libraryfortran.build 2024-02-05 07:37:01,799 lib.installable.installable INFO     Building http_client 0.1.0 for [flangtrunknew,,/opt/compiler-explorer/clang-llvmflang-trunk-20240205,Linux,Debug,,,,]
Feb 05 08:37:02 librarybuilder libraryfortran.build ERROR: Invalid setting 'flang' is not a valid 'settings.compiler' value.
Feb 05 08:37:02 librarybuilder libraryfortran.build Possible values are ['clang', 'clang-hexagon', 'cshared', 'edg', 'ellcc', 'fortran', 'gcc', 'gccrs', 'llvmmos', 'rust', 'zigcxx']
Feb 05 08:37:02 librarybuilder libraryfortran.build Read "http://docs.conan.io/en/latest/faq/troubleshooting.html#error-invalid-setting"