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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> The reduction has been crawling for 5 days now. The input is down to 275MB, but the progress has slowed down a lot.

Ouch... I don't know if it would already be possible to share it (offline with me only if needed) or does it still contain Google-internal code? Then I could at least have a look with a debugger and see if I can "guess" what is happening

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