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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

This change seems to impact the path add for #108015 which was explicitly related to vtable mangling, are we sure we're not regressing that bug?

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