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

lib/Bitcode/Reader/BitReader.cpp

@@ -16,6 +16,7 @@
 #include <string>
 
 using namespace llvm;
+using std::error_code;
 
 /* Builds a module from the bitcode in the specified memory buffer, returning a
    reference to the module via the OutModule parameter. Returns 0 on success.