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-size/llvm-size.cpp

@@ -30,6 +30,7 @@
 #include <system_error>
 using namespace llvm;
 using namespace object;
+using std::error_code;
 
 enum OutputFormatTy {berkeley, sysv};
 static cl::opt<OutputFormatTy>