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

tools/llvm-dis/llvm-dis.cpp

@@ -34,6 +34,7 @@
 #include "llvm/Support/ToolOutputFile.h"
 #include <system_error>
 using namespace llvm;
+using std::error_code;
 
 static cl::opt<std::string>
 InputFilename(cl::Positional, cl::desc("<input bitcode>"), cl::init("-"));