[clang] [llvm] [clang] Polymorphic Cleanup type is moved despite not being POD types (PR #156607)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

I've reviewed other changes here recently (#152866).  The code is playing some weird games with memory allocation, using an std::vector as an arena.  Probably there's some better way to write this.  Strictly speaking, it's undefined behavior to memmove a polymorphic class.  But this is how it works for now.  Maybe worth leaving a comment explaining this.

Otherwise LGTM

https://github.com/llvm/llvm-project/pull/156607