https://apt.llvm.org/

$ sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)"
$ clang-9 -v

# これ以降は，いらなかったみたい

$ curl -OL https://apt.llvm.org/llvm-snapshot.gpg.key
$ sudo apt-key add llvm-snapshot.gpg.key
$ sudo apt update
$ sudo apt install clang-9 lldb-9 lld-9

↓ 故障中