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
ln -s /usr/lib/llvm-11/bin/clang /usr/bin/c++
apt update
apt install llvm-11 clang-11 libclang-11-dev liblld-11-dev
apt install git cmake
git clone https://github.com/ziglang/zig.git
git checkout 0.6.0
mkdir build
cd build
cmake ..
make install
./build/bin/zig build --prefix $(pwd)/stage2 -Denable-llvm