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/tools/macho-dump/macho-dump.cpp b/llvm/tools/macho-dump/macho-dump.cpp
index e7575f0..13e8a4b 100644
--- a/llvm/tools/macho-dump/macho-dump.cpp
+++ b/llvm/tools/macho-dump/macho-dump.cpp
@@ -23,6 +23,7 @@
 #include <system_error>
 using namespace llvm;
 using namespace llvm::object;
+using std::error_code;
 
 static cl::opt<std::string>
 InputFile(cl::Positional, cl::desc("<input file>"), cl::init("-"));