Fix build errors after ceb9379a

For some reason some builds dont like the arrow operator access. using the deref then access should fix the issue.

/home/buildbots/ppc64le-flang-mlir-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include/llvm/ADT/iterator.h:171:34: error: taking the address of a temporary object of type 'llvm::StringRef' [-Waddress-of-temporary]
  PointerT operator->() { return &static_cast<DerivedT *>(this)->operator*(); }
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/buildbots/ppc64le-flang-mlir-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/llvm/include/llvm/ADT/StringExtras.h:387:13: note: in instantiation of member function 'llvm::iterator_facade_base<llvm::mapped_iterator<mlir::tblgen::TypeParameter *, (lambda at /home/buildbots/ppc64le-flang-mlir-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:414:19), llvm::StringRef>, std::random_access_iterator_tag, llvm::StringRef, long, llvm::StringRef *, llvm::StringRef &>::operator->' requested here
    Len += I->size();