[C++] Multiple definitions of std::hash<FieldPath> in unity builds

[benibus]

Describe the bug, including details regarding any error messages, version, and platform.

Discovered in an in-progress PR (#36073), but this will likely affect others soon if it hasn't already. In this case, it involves arrow/ipc/dictionary.cc and arrow/compute/kernels/vector_sort.cc.

Example error (from java-jni-manylinux-2014):

FAILED: src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx.o 
/usr/local/bin/ccache /opt/rh/devtoolset-10/root/usr/bin/c++ -DARROW_EXPORTING -DARROW_HAVE_RUNTIME_AVX2 -DARROW_HAVE_RUNTIME_AVX512 -DARROW_HAVE_RUNTIME_BMI2 -DARROW_HAVE_RUNTIME_SSE4_2 -DARROW_HAVE_SSE4_2 -DARROW_S3_HAS_CRT -DARROW_WITH_BACKTRACE -DARROW_WITH_LZ4 -DARROW_WITH_RE2 -DARROW_WITH_SNAPPY -DARROW_WITH_TIMING_TESTS -DARROW_WITH_UTF8PROC -DARROW_WITH_ZLIB -DARROW_WITH_ZSTD -DAWS_MQTT_WITH_WEBSOCKETS -DAWS_SDK_VERSION_MAJOR=1 -DAWS_SDK_VERSION_MINOR=11 -DAWS_SDK_VERSION_PATCH=4 -DAWS_USE_EPOLL -DCURL_STATICLIB -DURI_STATIC_BUILD -DUTF8PROC_STATIC -I/build/cpp/src -I/arrow/cpp/src -I/arrow/cpp/src/generated -isystem /arrow/cpp/thirdparty/flatbuffers/include -isystem /arrow/cpp/thirdparty/hadoop/include -isystem /opt/vcpkg/installed/amd64-linux-static-release/include -isystem /build/cpp/orc_ep-install/include -isystem /opt/vcpkg/installed/amd64-linux-static-release/share/rapidjson/../../include -isystem /build/cpp/jemalloc_ep-prefix/src -Wno-noexcept-type -Wno-subobject-linkage  -fdiagnostics-color=always  -Wall -fno-semantic-interposition -msse4.2  -O3 -DNDEBUG -O2 -ftree-vectorize  -fPIC -pthread -DS2N_KYBER512R3_AVX2_BMI2 -DS2N_STACKTRACE -DS2N_CPUID_AVAILABLE -DS2N_FEATURES_AVAILABLE -fPIC -DS2N_FALL_THROUGH_SUPPORTED -DS2N___RESTRICT__SUPPORTED -DS2N_MADVISE_SUPPORTED -DS2N_CLONE_SUPPORTED -std=c++17 -MD -MT src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx.o -MF src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx.o.d -o src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx.o -c /build/cpp/src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx
In file included from /build/cpp/src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx:15:
/arrow/cpp/src/arrow/ipc/dictionary.cc:40:8: error: redefinition of ‘struct std::hash<arrow::FieldPath>’
   40 | struct hash<arrow::FieldPath> {
      |        ^~~~~~~~~~~~~~~~~~~~~~
In file included from /build/cpp/src/arrow/CMakeFiles/arrow_objlib.dir/Unity/unity_23_cxx.cxx:3:
/arrow/cpp/src/arrow/compute/kernels/vector_sort.cc:24:13: note: previous definition of ‘struct std::hash<arrow::FieldPath>’
   24 | struct std::hash<arrow::FieldPath> : public arrow::FieldPath::Hash {};
      |             ^~~~~~~~~~~~~~~~~~~~~~

Job: https://github.com/apache/arrow/actions/runs/5337702550/jobs/9674081817 (zip)

Component(s)

C++