[clang] [clang] Upstream `clang::CodeGen::getConstantSignedPointer` (PR #154453)

[Ahmed Bougacha via cfe-commits]

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

We should be able to eventually remove this one, `getPointerAuthDeclDiscriminator`, and `getPointerAuthTypeDiscriminator`, which gradually got picked up, it looks like.  For this one specifically, we'd want to have a way of doing the null->zero translation in `ConstantPtrAuth::get` that we do today in `getConstantSignedPointer`.  But in the meantime LGTM

https://github.com/llvm/llvm-project/pull/154453