instructions on building Clang in README

[zjin-lcf]

Would you add back the instructions on building Clang from source in README.md ?

Downloading:

git clone -b release/14.x https://github.com/llvm/llvm-project.git
cd llvm-project/llvm/projects
git clone -b llvm_release_140 https://github.com/KhronosGroup/SPIRV-LLVM-Translator.git
Building:

cd llvm-project/llvm
mkdir build
cd build
cmake .. -DLLVM_ENABLE_PROJECTS="clang;openmp"
-DCMAKE_INSTALL_PREFIX=${LLVM_INSTALL_DIR}
make
make install