[clang] [llvm] [IR] Add ConstantExpr::getPtrAdd() (PR #181365)

[Aiden Grossman via cfe-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM.

Double checked that you hit all the users of `ConstantExpr::getGetElementPtr` for `i8`, and it seems like it.

https://github.com/llvm/llvm-project/pull/181365