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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> @emaxx-google, @ilya-biryukov, do you think we can move forward even though don't have a reproducer yet? It feels we are stuck on that one case on a downstream client -- worst case maybe the devs can work it around by changing source code for that one case?

Yes, I'm in favor of this. I feel it, more or less, doesn't make sense to wait for a downstream without a reproducer without a DDL ...

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