Try to fix MSVC build.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@104371 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/ArgList.cpp

@@ -16,6 +16,7 @@
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/raw_ostream.h"
 
+using namespace clang;
 using namespace clang::driver;
 
 void arg_iterator::SkipToNextArg() {