[clang] [Serialization] Fix lazy template loading (PR #133057)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

I don't think we should merge a partial state of this PR for two reasons: 1. The three patches alone don't actually bring much benefit. When I measured in ROOT, we definitely needed the (currently) problematic one for lazy template loading to become really effective. 2. As commented before, I suspect that the patch only reveals a problem that is already there right now. We manage to trigger it with that change, but I think it must be understood first before changing the baseline.

https://github.com/llvm/llvm-project/pull/133057