[llvm] [AArch64] Fix return address auth in swiftasync epilogues (PR #189484)

[John McCall via llvm-commits]

rjmccall wrote:

> pinging @rjmccall as this PR is outside of my wheelhouse, and I'm unclear how we've broken swift async - if this is an implementation step my assumption would have been that swift's downstream fork already did this and that needs to be upstreamed rather than reimplemented?

Yes, I agree — we should check our downstream fork. Are you up to do that?

https://github.com/llvm/llvm-project/pull/189484