[clang] [C++20] [Modules] Preserve DeductionGuideDecl if their parent are preserved (PR #222246)

[via cfe-commits]

dan0907 wrote:

This fixes the original repro, but the follow-up case here (https://github.com/llvm/llvm-project/issues/222002#issuecomment-5586406737) — a non-exported class used only inside a function template instantiated from the importing TU — still fails with this patch applied (verified locally). Is that in scope for this PR, or should it be tracked separately?

https://github.com/llvm/llvm-project/pull/222246