/usr/include/x86_64-linux-gnu/ is in libstdc++-8-dev 8-20180414-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-gnu/c++/
/usr/include/x86_64-linux-gnu/c++/8/
/usr/include/x86_64-linux-gnu/c++/8.0.1
/usr/include/x86_64-linux-gnu/c++/8/bits/
/usr/include/x86_64-linux-gnu/c++/8/bits/atomic_word.h
/usr/include/x86_64-linux-gnu/c++/8/bits/basic_file.h
/usr/include/x86_64-linux-gnu/c++/8/bits/c++allocator.h
/usr/include/x86_64-linux-gnu/c++/8/bits/c++config.h
/usr/include/x86_64-linux-gnu/c++/8/bits/c++io.h
/usr/include/x86_64-linux-gnu/c++/8/bits/c++locale.h
/usr/include/x86_64-linux-gnu/c++/8/bits/cpu_defines.h
/usr/include/x86_64-linux-gnu/c++/8/bits/ctype_base.h
/usr/include/x86_64-linux-gnu/c++/8/bits/ctype_inline.h
/usr/include/x86_64-linux-gnu/c++/8/bits/cxxabi_tweaks.h
/usr/include/x86_64-linux-gnu/c++/8/bits/error_constants.h
/usr/include/x86_64-linux-gnu/c++/8/bits/extc++.h
/usr/include/x86_64-linux-gnu/c++/8/bits/gthr-default.h
/usr/include/x86_64-linux-gnu/c++/8/bits/gthr-posix.h
/usr/include/x86_64-linux-gnu/c++/8/bits/gthr-single.h
/usr/include/x86_64-linux-gnu/c++/8/bits/gthr.h
/usr/include/x86_64-linux-gnu/c++/8/bits/messages_members.h
/usr/include/x86_64-linux-gnu/c++/8/bits/opt_random.h
/usr/include/x86_64-linux-gnu/c++/8/bits/os_defines.h
/usr/include/x86_64-linux-gnu/c++/8/bits/stdc++.h
/usr/include/x86_64-linux-gnu/c++/8/bits/stdtr1c++.h
/usr/include/x86_64-linux-gnu/c++/8/bits/time_members.h
/usr/include/x86_64-linux-gnu/c++/8/ext/
/usr/include/x86_64-linux-gnu/c++/8/ext/opt_random.h