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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210835 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-objdump/llvm-objdump.h

@@ -16,6 +16,7 @@
 #include "llvm/Support/StringRefMemoryObject.h"
 
 namespace llvm {
+using std::error_code;
 
 namespace object {
   class COFFObjectFile;