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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> But I don't think that need to be tested by google. It is just an new API.

To clarify, what needs testing is the new API plus this PR on top to see if it addresses the problem reported by @alexfh, and if there are others afterwards.

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