In the default LLVM provided by Apple in macOS 10.13, High Sierra, is OpenMP finally available? It has been available in main LLVM for more than a year now.
(another way to ask the question might be what version of LLVM is the new Apple LLVM based on)
Standard Apple's clang supports OpenMP. They just disabled the driver option. But you can use the frontend option instead this way: clang -Xclang -fopenmp <you_program> -I <path to omp.h> -L <path to libomp.dylib> -lomp
Also, you need to set DYLD_LIBRARY_PATH environmental variable: export DYLD_LIBRARY_PATH=<path to libomp.dylib>
How to get/build libomp.
$ cd
$ svn co http://llvm.org/svn/llvm-project/openmp/trunk libomp
$ cd libomp
$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=~/libomp/openmp
$ make && make install
After this directory ~/libomp/openmp will have 2 subdirs: include and lib, which should be used as the path to omp.h and the path to libomp.dylib correspondingly.
See also my answer to Is C++ compilable with OpenMP and boost on MacOS?