[mlir] Fix 'StringSet' may not intend to support class template argument deduction (NFC)

/llvm-project/mlir/tools/mlir-tblgen/OmpOpGen.cpp:202:3:
error: 'StringSet' may not intend to support class template argument deduction [-Werror,-Wctad-maybe-unsupported]
  llvm::StringSet superClasses;
  ^
/llvm-project/llvm/include/llvm/ADT/StringSet.h:23:7: note: add a deduction guide to suppress this warning
class StringSet : public StringMap<std::nullopt_t, AllocatorTy> {
      ^

Author: DamonFool

mlir/tools/mlir-tblgen/OmpOpGen.cpp

@@ -199,7 +199,7 @@ static StringRef translateArgumentType(ArrayRef<SMLoc> loc, StringInit *name,
                                        Init *init, int &nest, int &rank) {
   Record *def = cast<DefInit>(init)->getDef();
 
-  llvm::StringSet superClasses;
+  llvm::StringSet<> superClasses;
   for (auto [sc, _] : def->getSuperClasses())
     superClasses.insert(sc->getNameInitAsString());