With the new SVE stack layout, we now need to provide a Darwin variant for all the calling conventions based on the main AAPCS CSR save order.
This also changes APCS_SwiftError to have a Darwin and a non-Darwin version, assuming it could be used on other platforms these days, and restricts the AArch64_CXX_TLS calling convention to Darwin.
Should we be returning Darwin anything from here if we're moving all of the Darwin-specific code to a new function? Is this actually Darwin-specific?