The libclc package contains library requirements of the OpenCL C programming language (providing header files but no libraries).
The libclc package is part of the main LLVM tarball. You can either reuse the extracted directory, or remove it then re-extract the tarball again.
If you're reusing the previously extracted LLVM directory from the LLVM build, remove the build directory first before proceeding:
rm -rf build
Install libclc by running the following commands:
mkdir build &&
cd build &&
cmake -D CMAKE_INSTALL_PREFIX=/usr \
-D CMAKE_BUILD_TYPE=Release \
-G Ninja ../libclc &&
ninja
Now, as the root user:
ninja install
For a full list of options and values, make sure you are in a build directory, then run cmake -LAH ...