[clang] [Clang][CodeGen] Use ptrtoaddr for pointer diff (PR #210729)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Does this interact with #196392 ?  Should we prefer to continue using ptrtoint instead of ptrtoaddr in `-fdefined-pointer-subtraction` mode?

https://github.com/llvm/llvm-project/pull/210729