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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> After manually cleaning up the ~10KB that CVise could squeeze out of the initial 400+MB, I got this:

Thanks for your efforts, very useful! That reduced example actually crashes on `main` already, I opened https://github.com/llvm/llvm-project/issues/170084 (after some more simplifications). If your full internal code is currently compiling without problems, it is likely that this PR "just" exposes the issue which git-bisect says was introduced by 91cdd35008e9ab32dffb7e401cdd7313b3461892 on August 9.

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