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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

I'm not so sure it's about conversion function templates in particular: if it's really commit 658d55ba1117a029f37f51a3072585a1fd9fc59f causing the problem (@emaxx-google if you happen to have some time, it would be great to confirm reverting that change also avoids the issues in the original case), then there should only be a change for `ClassTemplatePartialSpecializationDecl` and `VarTemplatePartialSpecializationDecl`. In the reduced reproducer (https://github.com/llvm/llvm-project/pull/133057#issuecomment-2886816972), I only see a single partial specialization of `struct vy<ub*>`. To me the connection to the conversion operator of `me` is very unclear, but I'm definitely not an expert in Clang's template instantiation machinery...

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