[Presburger] Cheat around old versions of clang not doing NRVO when there's a...

[Presburger] Cheat around old versions of clang not doing NRVO when there's a derived-to-base cast in the way

Should be NFC. We can just do the base conversion manually and avoid
warnings about it. Clang before Clang 13 didn't implement P1825 and
complains:

mlir/lib/Analysis/Presburger/IntegerRelation.cpp:226:10: warning: local variable 'result' will be copied
      despite being returned by name [-Wreturn-std-move]
  return result;
         ^~~~~~
mlir/lib/Analysis/Presburger/IntegerRelation.cpp:226:10: note: call 'std::move' explicitly to avoid copying
  return result;
         ^~~~~~
         std::move(result)