4.4.0-81.104) is not available, but a newer one is (4.4.0-142.168). We redirected you there.
/usr/lib/llvm-6.0/build/Debug+Asserts 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/build/..