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/lib/IRReader/IRReader.cpp b/llvm/lib/IRReader/IRReader.cpp
index 3e928d4..e170a3c 100644
--- a/llvm/lib/IRReader/IRReader.cpp
+++ b/llvm/lib/IRReader/IRReader.cpp
@@ -21,6 +21,7 @@
 #include <system_error>
 
 using namespace llvm;
+using std::error_code;
 
 namespace llvm {
   extern bool TimePassesIsEnabled;