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

unittests/Transforms/DebugIR/DebugIR.cpp

@@ -18,6 +18,7 @@
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/Module.h"
+#include "llvm/Support/Errc.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/Path.h"
@@ -58,7 +59,7 @@ bool removeIfExists(StringRef Path) {
   // This is an approximation, on error we don't know in general if the file
   // existed or not.
   std::error_code EC = sys::fs::remove(Path, false);
-  return EC != std::errc::no_such_file_or_directory;
+  return EC != llvm::errc::no_such_file_or_directory;
 }
 
 char * current_dir() {