LLVM 10 headers incompatible with gcc 7

[insertinterestingnamehere]

In LLVM 10 the LLVM headers that we're using for command line parsing rely on the fact that std::char_traits<char>::length can be used in constexpr contexts in C++17. Gcc 7 wrongly complains about that. Though this isn't our bug, we may want to warn users about it (or even error out) if they attempt to build with that package combination.