/usr/lib/llvm-6.0/include/llvm-c is in llvm-6.0-dev 1:6.0-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/llvm-6.0/include/../../../include/llvm-c-6.0/llvm-c
/usr/lib/llvm-6.0/include/../../../include/llvm-c-6.0/llvm-c