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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> > Invested some time today and I think the fix is #154158. It works for the reproducer posted in [#133057 (comment)](https://github.com/llvm/llvm-project/pull/133057#issuecomment-2886816972), @ilya-biryukov or @emaxx-google if you have some time, would it be possible to test the two PRs together in your infrastructure?
> 
> Unfortunately after repeating the testing on the google monorepo with these two changes cherry-picked, there's a lot of errors observed. Minimizing examples would take a few days, but to name some error messages:
> 
>     * `'std::tuple<std::string_view>::operator=' from module '...' is not present in definition of 'std::tuple<std::string_view>' provided earlier`
> 
>     * `'std::pair<std::string, std::string>::operator=' from module '...' is not present in definition of 'std::pair<std::string, std::string>' provided earlier`

Hm, and that's after or instead of the previous problems. Ie did https://github.com/llvm/llvm-project/pull/154158 fix a problem and we get past that point, now seeing "later" issues?

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