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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> 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.

Do you mean that no large increases in the memory use or that things are not getting faster either?
If it's the latter, I would vouch for keeping the code as-is to avoid adding extra complexity.
If this does lead to significant performance wins, though, it's a different story.

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