wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
sudo apt-add-repository "<relevant 'deb http://...' text from https://apt.llvm.org>"
sudo apt update
sudo apt install clang-<latest-version>
sudo apt install libclang-<latest-version>-dev
# may need to install llvm-<latest-version>, llvm-<latest-version>-runtime
Also may have to fix what llvm-config
points to. Do this.