Don't use std::errc.

As noted on Errc.h:

// * std::errc is just marked with is_error_condition_enum. This means that
//   common patters like AnErrorCode == errc::no_such_file_or_directory take
//   4 virtual calls instead of two comparisons.

And on some libstdc++ those virtual functions conclude that

------------------------
int main() {
  std::error_code foo = std::make_error_code(std::errc::no_such_file_or_directory);
  return foo == std::errc::no_such_file_or_directory;
}
-------------------------

should exit with 0.

llvm-svn: 239685