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

llvm/lib/Bitcode/Reader/BitcodeReader.h

@@ -26,6 +26,7 @@
 #include <vector>
 
 namespace llvm {
+using std::error_code;
   class MemoryBuffer;
   class LLVMContext;