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

[via cfe-commits]

tcwzxx wrote:

Given that the mangleCXXRecordDecl function is used outside of the `MangleVTable` , I think the ABI compatibility logic should be moved to mangleCXXCtorVTable

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