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/gold/gold-plugin.cpp

@@ -44,6 +44,7 @@
 #endif
 
 using namespace llvm;
+using std::error_code;
 
 namespace {
   ld_plugin_status discard_message(int level, const char *format, ...) {