Removal of libllvm4.0 from disco/universe

[Dan Kegel]

For what it's worth, I just forward-ported 3.9 from xenial to disco
because the alternative was reformatting a bazillion lines of
source code.  It wasn't too hard, just had to do 'apt source clang-format'
on an ubuntu 18.04 box, transfer the source to a 19.04 box,
install gcc 7 and make three little changes to the package, to wit:
http://kegel.com/linux/llvm-ubu1904.patch

There's probably a PPA somewhere with older versions of clang-format,
but if there isn't, I could create one.
- Dan