Building on D90868, this introduces a new "ptrauth" operand bundle to be used in call/invoke. At the IR level, it's semantically equivalent to an @llvm.ptrauth.auth followed by an indirect call, but it additionally provides hardening guarantees, by preventing the intermediate raw pointer from being exposed.
This mostly adds the IR definition, verifier checks, and support in a couple of general helper functions. clang IRGen and backend support will come separately.
Note that we'll eventually want to support this bundle in indirectbr as well, for similar reasons. indirectbr currently doesn't allow bundles at all, and the IR data structures need some rejiggering.