[clang] [Serialization] Don't read all declaration id eagerly when merge the tables (PR #95506)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I feel the increased complexity does not seem like a good trade-off if it does not significantly improve performance.
However I don't feel too strongly about it, so I suggest you get a second opinion if you feel strongly that we should land this.

https://github.com/llvm/llvm-project/pull/95506