sudo apt-get install software-properties-common 
sudo add-apt-repository ppa:jonathonf/gcc-9.0 
sudo apt-get update 
sudo apt-get install gcc-9

sudo apt-get install build-essential gcc-9-plugin-dev clang ninja-build cmake libncurses5-dev libelf-dev libssl-dev flex bison bc git pigz
// git clone https://github.com/llvm/llvm-project.git
cd llvm-project
git checkout release/10.x
mkdir build
cd build

# cmake -G Ninja -DLLVM_ENABLE_PROJECTS='clang;lld;compiler-rt' \
-DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_WARNINGS=OFF \
-DCMAKE_INSTALL_PREFIX=/usr/local/llvm-10 ../llvm

cmake -G Ninja -DLLVM_ENABLE_PROJECTS='clang;lld;compiler-rt' -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_WARNINGS=OFF -DCMAKE_INSTALL_PREFIX=/usr/local/llvm-10 ../llvm

ninja
sudo ninja install

cmake -DLLVM_ENABLE_PROJECTS='clang;lld;compiler-rt' -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_WARNINGS=OFF -DCMAKE_INSTALL_PREFIX=/usr/local/llvm-10 ../llvm
cmake --build .
mkdir  ./docs/ocamldoc
mkdir  ./docs/ocamldoc/html
make 
sudo make install

export PATH=/usr/local/llvm-10/bin:$PATH

OR

export PATH=/home/wenhui/llvm-10.0.0.src/build/bin:$PATH

sudo ln -s  /usr/local/llvm-10/bin/clang /usr/bin/clang

// cp .config 
make menuconfig LLVM=1
make -j4 LLVM=1
sudo make modules_install LLVM=1
sudo make install LLVM=1
sudo update-grub2
sudo reboot