[llvm-dwp] Use cl:: instead of using namespace cl

`using namespace cl` makes llvm::cl::Optional (in Support/CommandLine.h) visible which will cause ambiguity when unqualified `Optional` is looked up (can also refer to llvm::Optional).

cl:: is used much more than `using namespace cl`, so let's not use the latter.

Also append \n to the argument of cl::ParseCommandLineOptions

llvm-svn: 341584