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.
Differential Revision: https://reviews.llvm.org/D73805
