看 https://github.com/Valloric/YouCompleteMe#c-family-semantic-completion installation部分(安装之前看以下部分)
注意user guide写的：For other build systems, check out .ycm_extra_conf.py below. Note that experimental clangd-based completer doesn’t support this option. So it (and anything after this point) only applies to libclang-based completer 所以尽量用libclang-based编绎 出问题了看user guide---->C-family Semantic Completion部分，大部分可以解决。 .ycm_extra_conf.py文件如果不行，用下面这个(user guide部分的)wget命令获得 https://raw.githubusercontent.com/Valloric/ycmd/66030cd94299114ae316796f3cad181cac8a007c/.ycm_extra_conf.py
