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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 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 I'm wrong?

Yeah, I think this patch may be conceptually good except the extra memory using. But benchmarking, I tried it locally but didn't find observable effects. So I'd like to land this after 19's branching to give it more baking time.

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