llvm::cl::ExpandReponseFiles always returns false

I stumbled upon a minor bug in llvm/lib/CommandLine.cpp.
Due to incorrect return value initialization,
llvm::cl::ExpandResponseFiles() always returns false,
even if all rsp files get expanded correctly.

All callers (llvm/clang) ignore this return value, but I'd
appreciate if someone would fix that anyway.

Thanks!

Should be fixed in r196306.