[Bug] Clang 19 compilation failure

使用 workspace-automation 命令在 Fedora 41 上重现失败，并检查 third_party/flatbuffers/include/flatbuffers/flatbuffers.h，尤其是第 834、1002 和 1874 行附近的赋值。通过 external/src/flatbuffers/src/reflection.cpp 检查相关的编译路径，并确认 Firebase C++ SDK 能够在 Clang 19 下成功构建，且不会出现 deleted-operator 错误。

[REQUIRED] Please fill in the following fields:

• Pre-built SDK from the website or open-source from this repo: source
• Firebase C++ SDK version: v12.5.0
• Problematic Firebase Component: third_party - flatbuffers
• Other Firebase Components in use: Cloud, Auth, Storage, common
• Platform you are using the C++ SDK on: Ubuntu 20 to Linux Fedora 41
• Platform you are targeting: desktop, embedded Linux (Yocto)

[REQUIRED] Please describe the issue here:

SDK fails to build with Clang 19 (Fedora 41). With minimal patch it builds fine on Clang 10 (Ubuntu 20).

This error will be repeated Nx depending on CPU count used in build:

In file included from /mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/src/reflection.cpp:17:
In file included from /mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/include/flatbuffers/reflection.h:25:
In file included from /mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/include/flatbuffers/reflection_generated.h:7:
/mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/include/flatbuffers/flatbuffers.h:1874:12: error: overload resolution selected deleted operator '='
 1874 |       buf_ = other.buf_;
      |       ~~~~ ^ ~~~~~~~~~~
/mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/include/flatbuffers/flatbuffers.h:1002:44: note: candidate function has been explicitly deleted
 1002 |   FLATBUFFERS_DELETE_FUNC(vector_downward &operator=(const vector_downward &))
      |                                            ^
/mnt/raid10/workspace-automation/app/firebase-cpp-sdk/cmake-build-release/external/src/flatbuffers/include/flatbuffers/flatbuffers.h:834:20: note: candidate function not viable: expects an rvalue for 1st argument
  834 |   vector_downward &operator=(vector_downward &&other) {
      |                    ^         ~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Steps to reproduce:

Fedora 41

git clone https://github.com/meta-flutter/workspace-automation
cd workspace-automation
./flutter_workspace.py --enable=firebase-cpp-sdk