$ sudo apt-add-repository "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main"
$ sudo apt-get update
$ sudo apt-get install clang-4.0 lldb-4.0

$ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
$ sudo apt-add-repository "deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-3.9 main"
$ sudo apt -o Acquire::AllowInsecureRepositories=true update
$ sudo apt-get install clang-3.9 lldb-3.9