These instructions assert a clean install of Ubuntu 20.04 with no previous LLVM or C compiler installed.
apt update
apt install wget lsb-release software-properties-common
bash -c "$(wget -O - https://apt.llvm.org/llvm.sh 11)"
ln -s /usr/lib/llvm-11/bin/lld /usr/bin/lld
ln -s /usr/lib/llvm-11/bin/clang /usr/bin/clang