[clang] Use `llvm::SmallVector` instead of `OwningArrayRef` in `VTableLayout`. (PR #168768)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon approved this pull request.

I think this makes sense. Please give @cor3ntin a couple of days in case there is any more feedback

https://github.com/llvm/llvm-project/pull/168768