[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:

(sorry for the noise, I see the above references are PRs rather than the issue # so they're all the same thing :D)

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