/usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6 is in libstdc++6-4.9-dbg 4.9.2-10+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6.0.20
/usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6.0.20