[clang] [clang] Fix self-capturing `__block` variables (PR #89475)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

Serialization related change looks trivial and good. But I feel better to leave the formal approval to CG part reviewers.

https://github.com/llvm/llvm-project/pull/89475