Kernel/meta_enum.h does not work with gcc 11

The builds of LHCb with gcc 11 on lhcb-lcg-dev4 fail massively because of an issue with meta_enum.h. See https://lhcb-nightlies.web.cern.ch/nightly/lhcb-lcg-dev4/1758/LHCb/x86_64_v2-centos7-gcc11-opt/build#Calo/CaloInterfaces_5

According to the upstream docs a similar error was seen on clang 6 (due to string_view not being constexpr where it should be). Starting from the suggested compiler explorer link I could verify that the problem exists for gcc 11.1, 11.2 and trunk: https://godbolt.org/z/Ton91fMfo