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

[Vassil Vassilev via cfe-commits]

vgvassilev 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.

@hahnjo, if we have a working reproducer could we see if @zygoloid's comment helps us:

> If this issue is indeed specific to conversion function templates, perhaps a path forward would be to disable some part of the lazy loading for only those templates to unblock this patch while the reason for the problem is being investigated?

That would mean that we exclude from lazy serialization the templated conversion functions.

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