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/lib/DebugInfo/DWARFContext.cpp b/llvm/lib/DebugInfo/DWARFContext.cpp
index e52e8af..110fd81 100644
--- a/llvm/lib/DebugInfo/DWARFContext.cpp
+++ b/llvm/lib/DebugInfo/DWARFContext.cpp
@@ -21,6 +21,7 @@
 using namespace llvm;
 using namespace dwarf;
 using namespace object;
+using std::error_code;
 
 #define DEBUG_TYPE "dwarf"