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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@ChuanqiXu9 are you still planning to chase this given that fixing the hashing function would fix performance for the aforementioned patch?

The trade-off we are making here is hard to assess without benchmarks that show how much latency we win and how much more memory we spend. I am not sure if doing the benchmarks would be high on anyone's priority list, but maybe?

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