We need to -isystem the libcxx header files in order for clang-tidy to ignore them, as the Nix clang toolchain isn't doing that automatically.
Change-Id: I05b9e9bd522de4c0e2ad543214f6bf6ab66a306b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1359
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI