[C++] mockfs.cc fails to build with -DCMAKE_CXX_STANDARD=23 #46998

cmake ../cpp/ -DARROW_FILESYSTEM=ON -DARROW_PARQUET=ON -DARROW_POSITION_INDEPENDENT_CODE=ON -DARROW_BUILD_SHARED=ON -DARROW_WITH_ZLIB=ON -DZLIB_SOURCE=SYSTEM -DARROW_DEPENDENCY_SOURCE=BUNDLED -DCMAKE_CXX_STANDARD=23

Apple clang version 17.0.0 (clang-1700.0.13.5)
Target: x86_64-apple-darwin24.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

/Library/Developer/CommandLineTools/SDKs/MacOSX15.5.sdk/usr/include/c++/v1/__memory/unique_ptr.h:79:19: error: invalid application of 'sizeof' to an incomplete type 'arrow::fs::internal::(anonymous namespace)::Entry'
   79 |     static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
      |                   ^~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX15.5.sdk/usr/include/c++/v1/__memory/unique_ptr.h:293:7: note: in instantiation of member function 'std::default_delete<arrow::fs::internal::(anonymous namespace)::Entry>::operator()' requested here
  293 |       __ptr_.second()(__tmp);
      |       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.5.sdk/usr/include/c++/v1/__memory/unique_ptr.h:233:5: note: in instantiation of member function 'std::unique_ptr<arrow::fs::internal::(anonymous namespace)::Entry>::reset' requested here
  233 |     reset(__u.release());
      |     ^
/Users/zhjwpku/zhjwpku/arrow/cpp/src/arrow/filesystem/mockfs.cc:113:16: note: in instantiation of member function 'std::unique_ptr<arrow::fs::internal::(anonymous namespace)::Entry>::operator=' requested here
  113 |     entries[s] = std::move(entry);
      |                ^
/Users/zhjwpku/zhjwpku/arrow/cpp/src/arrow/filesystem/mockfs.cc:57:7: note: forward declaration of 'arrow::fs::internal::(anonymous namespace)::Entry'
   57 | class Entry;
      |       ^
1 error generated.