Removal of libllvm4.0 from disco/universe

[Dan Kegel]

Quicker approach:
realizing that the package you want is gone but not forgotten :-),
download the debs from
https://launchpad.net/ubuntu/disco/amd64/clang-format-4.0/1:4.0.1-10build1
and
https://launchpad.net/ubuntu/cosmic/amd64/libllvm4.0/1:4.0.1-10build1
and install them with
  sudo dpkg -i libllvm4.0_4.0.1-10build1_amd64.deb
clang-format-4.0_4.0.1-10build1_amd64.deb
and Bob's your uncle.
- Dan