[clang] [clang-repl] Unlink the withdrawn partial translation unit on Undo (PR #213235)

[Emery Conrad via cfe-commits]

conrade-ctc wrote:

@vgvassilev @Vipul-Cariappa, makes sense to me. Happy to try to drive that, I'll try to follow this: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches, but I suspect there will be some permissions that I don't have!

https://github.com/llvm/llvm-project/pull/213235