[clang] [ItaniumMangle] Make sure class types are added to the dictionary of substitution candidates when compiling for older ABIs (PR #138947)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

> Can you confirm that the intent here is to restore mangler behavior to what it was prior to #132401, making the pair of PRs ABI-neutral?

Yes, exactly. This PR intends to restore the behavior prior to #132401 for older ABIs.

https://github.com/llvm/llvm-project/pull/138947