Remove all uses of 'using std::error_code' from headers.

llvm-svn: 210866
diff --git a/llvm/tools/llvm-objdump/MachODump.cpp b/llvm/tools/llvm-objdump/MachODump.cpp
index eaad96c..cba7318 100644
--- a/llvm/tools/llvm-objdump/MachODump.cpp
+++ b/llvm/tools/llvm-objdump/MachODump.cpp
@@ -42,6 +42,7 @@
 #include <system_error>
 using namespace llvm;
 using namespace object;
+using std::error_code;
 
 static cl::opt<bool>
   UseDbg("g", cl::desc("Print line information from debug info if available"));