[clang] [serialization] no transitive decl change (PR #92083)

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alexfh wrote:

Thank you for the analysis, Ilya!

Re: a possible resolution, I wonder why `llvm::hash_value` from llvm/ADT/Hashing.h is not used in `DenseSet`? In any case, if this can't be resolved quickly and needs a design discussion, it would be appropriate to revert the patch first.

https://github.com/llvm/llvm-project/pull/92083