add support for afl-clang-fast

[ddcc]

By default, the wrapper script will execute llvm-cov, which can be overriden by the environment variable AFL_COV_LLVM. As an example:

AFL_COV_LLVM=llvm-cov-8 afl-cov -d <outdir> -c <srcdir> -e "<binpath> < AFL_FILE" --clang --enable-branch-coverage