[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:

Ok, I went over this with Akira, and verified the discriminators for vtable pointers and vtable pointer slots have remained consistent. I think we should consider adding additional run commands to some of the existing pointer auth tests to explicitly run with abi-compatibility=4 and latest to ensure that both modes always produce the same discriminators. That's not necessary in this PR though.

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