[clang] [PAC][clang] Implement address/type discrimination for VTT vtable pointers (PR #205808)

[Kristina Bessonova via cfe-commits]

chbessonova wrote:

@ojhunt thank you, I added documentation and declared `_VTT` suffix as a constant in `PointerAuthOptions.h`.
I had to rebase and force push to resolve merge conflicts, new changes added in separate commits.

https://github.com/llvm/llvm-project/pull/205808