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

[Matheus Izvekov via cfe-commits]

mizvekov 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?

I a bit confused here as well. The description on the PR doesn't say this, but the patch seems to claim that https://github.com/llvm/llvm-project/pull/132401 fixed a bug here, but it was an ABI breaking change, so it wishes to preserve the bug for ABI < 19.

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