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

[Vassil Vassilev via cfe-commits]

vgvassilev 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? 🤔

Creduce-like tools are very bad at finding reject valid codes. @emaxx-google, can you paste your interestingness test?

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