[VFS] Use generic_category for errors generated from the VFS

[bnbarham]

Summary

Errors are generally checked in clients by comparing to the portable
error condition in std::errc, which will have the generic_category
(eg. std::errc::no_such_file_or_directory). While in practice these
are usually equivalent for the standard errno's, they are not in *all*
implementations. One such example is CentOS 7.

llvm/lib/Support/VirtualFileSystem.cpp

std::error_code incrementIter(bool IsFirstTime) {
     while (!IterList.empty()) {
       CurrentDirIter = IterList.back();
       IterList.pop_back();
       if (CurrentDirIter != directory_iterator())
         break; // found
     }
 
     if (IsFirstTime && CurrentDirIter == directory_iterator())
-      return std::error_code(static_cast<int>(errc::no_such_file_or_directory),
-                             std::system_category());
+      return errc::no_such_file_or_directory;
     return {};
   }
 
   std::error_code incrementDirIter(bool IsFirstTime) {
     assert((IsFirstTime || CurrentDirIter != directory_iterator()) &&
            "incrementing past end");
     std::error_code EC;
     if (!IsFirstTime)
if (Redirection != RedirectKind::RedirectOnly &&
         isFileNotFound(S.getError(), Result->E))
       return ExternalFS->dir_begin(Dir, EC);
 
     EC = S.getError();
     return {};
   }
 
   if (!S->isDirectory()) {
-    EC = std::error_code(static_cast<int>(errc::not_a_directory),
-                         std::system_category());
+    EC = errc::not_a_directory;
     return {};
   }
 
   // Create the appropriate directory iterator based on whether we found a
   // DirectoryRemapEntry or DirectoryEntry.
   directory_iterator RedirectIter;
   std::error_code RedirectEC;
   if (auto ExtRedirect = Result->getExternalRedirect()) {