[VFS] Use generic_category for errors generated from the VFS

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.

Differential Revision: https://reviews.llvm.org/D120299

Author: bnbarham

llvm/lib/Support/VirtualFileSystem.cpp

@@ -485,8 +485,7 @@ class CombiningDirIterImpl : public llvm::vfs::detail::DirIterImpl {
     }
 
     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 {};
   }
 
@@ -1285,8 +1284,7 @@ directory_iterator RedirectingFileSystem::dir_begin(const Twine &Dir,
   }
 
   if (!S->isDirectory()) {
-    EC = std::error_code(static_cast<int>(errc::not_a_directory),
-                         std::system_category());
+    EC = errc::not_a_directory;
     return {};
   }