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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> While I may not able to look into them in detail recently, it may be helpful to split this into seperate patches to review and to land.

I initially considered this, but @vgvassilev said in https://github.com/root-project/root/pull/17722#issuecomment-2706555950 he prefers a single PR, also for external testing.

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