[clang] [Serialization] Fix crash while lazy-loading template specializations (PR #150430)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> Currently running all regression tests to make sure nothing else breaks.

Yes, all seems to work fine.

> Historically, I believe we may have hit the same problem and our proposal was to have additional functions to iterate over already loaded specializations: https://reviews.llvm.org/D154328 We'll try to see if the change proposed here is actually the correct fix for the underlying problem.

It turns out the motivation is slightly different: We use the new methods in our `DeclUnloader` where we want to really avoid loading anything that we are in the process of removing.

https://github.com/llvm/llvm-project/pull/150430