[clang] [Serialization] Load Specializations Lazily (PR #76774)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> That'd mean that we "just" need to replace LazySpecializationInfo *LazySpecializations = nullptr; with the on-disk hash table approach. That would probably require centralizing that logic somewhere in the ASTReader (the way this PR does) but with minimal changes wrt D41416.

@vgvassilev Let me try to double check your advice. In you suggestion, you suggest to replace `LazySpecializationInfo *LazySpecializations` with an on-disk hash map from an integer (hash value for template args) to  LazySpecializationInfo in D41416 instead of another integer (DeclID, just like my patch)?

https://github.com/llvm/llvm-project/pull/76774