[clang] [Serialization] Storing DeclID separately (PR #95897)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@ilya-biryukov I think this is ready to review and test. I think this can mitigate the size increase with the DeclID change.

https://github.com/llvm/llvm-project/pull/95897