[clang] [llvm] [clang][Sema] Use DenseMap for SpecialMemberCache (NFC) (PR #221304)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Beside the FoldingSetNodeID size overhead, FoldingSet has slow lookup key (serialised into a FoldingSetNodeID), hash (external call, xxh3), and equality on a hit. However, it has faster growth/erase, but it's overall worse than DenseMap in this use case.

https://github.com/llvm/llvm-project/pull/221304