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

[Maksim Ivanov via cfe-commits]

emaxx-google wrote:

> I'm not so sure it's about conversion function templates in particular: if it's really commit [658d55b](https://github.com/llvm/llvm-project/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`.

I confirmed that reverting [658d55b] results in a successful compilation of the original google3 target.

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