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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Complete only needed partial specializations: It is unclear (to me) why this needs to be done "for safety", but this change significantly improves the effectiveness of lazy loading.

This comes from the logic: if we have a partial template specialization `A<int,  T,  U>` and we need a full specialization  for `A<int, double, double>`, we hope the partial specialization to be loaded

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