/usr/lib/llvm-3.9/include/llvm-c is in llvm-3.9-dev 1:3.9.1-19ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/llvm-3.9/include/../../../include/llvm-c-3.9/llvm-c
/usr/lib/llvm-3.9/include/../../../include/llvm-c-3.9/llvm-c