I had the same problem with Ubuntu-18.04. It seems that the system libLLVM-6.0.so is picked instead of the library which julia builder just compiled. It looks to me like a problem in the Makefile.
I was able to fix it by extending the LD_LIBRARY_PATH
# inside julia folder
find . -name libLLVM-6.0.so
# gives
# ./usr/lib/libLLVM-6.0.so
# ./deps/scratch/llvm-6.0.0/build_Release/lib/libLLVM-6.0.so
export LD_LIBRARY_PATH="$(pwd)/usr/lib/:$LD_LIBRARY_PATH"
make