[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:

Given that the original PR broke pointer auth discriminators for function pointers, can we verify that discriminators for vtable pointers and vtable pointer slots are still correct as well? I think what we might actually need is for mangling to be aware that it is being used for the purpose of constructing a pointer auth discriminator- (this would also make it possible for us to handle the problem of abi versions being included in the mangling of vtable pointer slots)

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