This adds support for tail calling callees with varargs, equivalent to how it is done in AArch64ISelLowering.
This only works for sibling calls, and does not add the necessary support for musttail with varargs. (See r345641 for equivalent ISelLowering support.) This should be implemented when we stop falling back on musttail.
Update call-translator-tail-call.ll to show that we can now tail call varargs.