Don't use 'using std::error_code' in include/llvm.

This should make sure that most new uses use the std prefix.

llvm-svn: 210835
diff --git a/llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp b/llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp
index df6d535..e86d93b 100644
--- a/llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp
+++ b/llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp
@@ -28,6 +28,7 @@
 #include <system_error>
 using namespace llvm;
 using namespace llvm::object;
+using std::error_code;
 
 static cl::list<std::string>
 InputFileList(cl::Positional, cl::ZeroOrMore,