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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> Here's a new reproducer, this time verifying that a semi-fresh tip-of-the-tree doesn't trigger the same error: https://pastebin.com/7tYfjazz. Apologies for the delay.

Thanks. I gave it a try, but I don't see any `use of overloaded operator '=' is ambiguous` error. In fact, there isn't a `operator =` in the sources at all. What am I missing? :thinking: 

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