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/bugpoint/ExecutionDriver.cpp

@@ -22,6 +22,7 @@
 #include <fstream>
 
 using namespace llvm;
+using std::error_code;
 
 namespace {
   // OutputType - Allow the user to specify the way code should be run, to test