[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)

mlir/lib/Analysis/Presburger/IntegerRelation.cpp

@@ -215,7 +215,7 @@ PresburgerRelation IntegerRelation::computeReprWithOnlyDivLocals() const {
                          IntegerPolyhedron(PresburgerSpace::getSetSpace(
                              /*numDims=*/copy.getNumVars() - numNonDivLocals)))
           .computeSymbolicIntegerLexMin();
-  PresburgerSet result =
+  PresburgerRelation result =
       lexminResult.lexmin.getDomain().unionSet(lexminResult.unboundedDomain);
 
   // The result set might lie in the wrong space -- all its ids are dims.