For those who builds llvm and clang for the first time:
git clone https://github.com/llvm/llvm-project
git checkout origin/release/9.x
cd llvm-project
put clang2nim directory under clang/tools/
add_clang_subdirectory(clang2nim)
mkdir build && cd build
cmake -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DLLVM_TARGETS_TO_BUILD=host -DLLVM_ENABLE_BINDINGS=OFF -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr/lib/x86_64-linux-gnu/ -G Ninja ../llvm && time cmake --build . -j8
cmake --install .
vim /etc/ld.so.conf.d/x86_64-linux-gnu
append /usr/lib/x86_64-linux-gnu/lib(where you install llvm and clang libraries)
ldconfig
cmake --build .
bin/clang2nim test.cpp