Installing clang 9 on ubuntu 18.04 LTS

1. Add llvm key to apt keys
   wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
2. Add llvm bionic clang 9 apt repo
   sudo add-apt-repository -u 'http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
3. Install clang 9
   sudo apt install clang-9