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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> > Great! In that case, I would like to proceed with landing the commits individually so they can be more easily reverted if needed (see discussion at the beginning of the PR), and then we can see if we can / want to backport for `release/22.x`. Please object if you see a problem with that plan.
> 
> As this patch doesn't add any new functionalities. I think we don't need to backport it.

No, it doesn't add new functionalities but it finally fixes the feature of lazy template loading to actually work. But backporting is not a requirement, we can continue to carry the patches downstream in ROOT.

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