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

llvm-svn: 210866
diff --git a/llvm/tools/llvm-objdump/COFFDump.cpp b/llvm/tools/llvm-objdump/COFFDump.cpp
index 7a0fa6e1..84799ec 100644
--- a/llvm/tools/llvm-objdump/COFFDump.cpp
+++ b/llvm/tools/llvm-objdump/COFFDump.cpp
@@ -29,6 +29,7 @@
 using namespace llvm;
 using namespace object;
 using namespace llvm::Win64EH;
+using std::error_code;
 
 // Returns the name of the unwind code.
 static StringRef getUnwindCodeTypeName(uint8_t Code) {