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

lib/Object/ELFObjectFile.cpp

@@ -15,6 +15,7 @@
 #include "llvm/Support/MathExtras.h"
 
 namespace llvm {
+using std::error_code;
 using namespace object;
 
 ErrorOr<ObjectFile *> ObjectFile::createELFObjectFile(MemoryBuffer *Obj,