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/Support/FileUtilities.cpp b/llvm/lib/Support/FileUtilities.cpp
index 68953d3..729e447 100644
--- a/llvm/lib/Support/FileUtilities.cpp
+++ b/llvm/lib/Support/FileUtilities.cpp
@@ -22,6 +22,7 @@
 #include <cstring>
 #include <system_error>
 using namespace llvm;
+using std::error_code;
 
 static bool isSignedChar(char C) {
   return (C == '+' || C == '-');