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

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

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

Could you elaborate on how to separate just the ABI compatibility logic from `mangleCXXRecordDecl`?

Wouldn't you have to move the code to `mangleCXXVTable` and `mangleCXXVTT` too? I'm not sure we want to duplicate code in multiple functions.

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