Try to fix MSVC build.

llvm-svn: 104371
diff --git a/clang/lib/Driver/ArgList.cpp b/clang/lib/Driver/ArgList.cpp
index 9ca710f..dbcc115 100644
--- a/clang/lib/Driver/ArgList.cpp
+++ b/clang/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() {