[lit] Fix internal shell's argv[0] handling.

 - At least on OS X, it is important for correct behavior of /bin/[ that argv[0]
   is passed as written, and not as the full executable path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189559 91177308-0d34-0410-b5e6-96231b3b80d8