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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> @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.

I replied to this suggestion in https://github.com/llvm/llvm-project/pull/133057#issuecomment-2889787645. That still stands, I don't see that anything changed since May.

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