Remove all uses of 'using std::error_code' from headers.

llvm-svn: 210866

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -26,6 +26,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
+using std::error_code;
 
 enum {
   SWITCH_INST_MAGIC = 0x4B5 // May 2012 => 1205 => Hex