The package from your package manager may come with a libLLVM.so, in which case you're all set. Notably, Xcode does not, so you'll want to install from Homebrew, which provides the shared library by default.
If you build from source yourself, you'll want to set the LLVM_BUILD_LLVM_DYLIB configuration variable when running CMake, according to the LLVM guide.
Finally, to find the library, you may need to set an environment variable or otherwise configure the dynamic linking search path. The environment variable might be called LD_LIBRARY_PATH or DYLD_LIBRARY_PATH.
Highlights: