Finishing touch for the std::error_code transition.

While std::error_code itself seems to work OK in all platforms, there
are few annoying differences with regards to the std::errc enumeration.

This patch adds a simple llvm enumeration, which will hopefully avoid build
breakages in other platforms and surprises as we get more uses of
std::error_code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210920 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/ErrorOr.h

@@ -99,7 +99,6 @@ public:
                                       std::is_error_condition_enum<E>::value,
                                   void *>::type = 0)
       : HasError(true) {
-    using std::make_error_code;
     new (getErrorStorage()) std::error_code(make_error_code(ErrorCode));
   }