[clang][ObjC][PAC] Include objective-c ptrauth abi in llvm21

[ojhunt]

We need to include the objc ptrauth ABI changes as part of the llvm21 pointer auth ABI adoption.

/cherry-pick 451a9ce

[llvmbot]

Failed to cherry-pick: 451a9ce

https://github.com/llvm/llvm-project/actions/runs/16975354698

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[ojhunt]

I am a muppet and had already merged this