[AArch64] Implement passing SVE vectors by ref for AAPCS.

Summary:
This patch implements the part of the calling convention
where SVE Vectors are passed by reference. This means the
caller must allocate stack space for these objects and
pass the address to the callee.

Reviewers: efriedma, rovka, cameron.mcinally, c-rhodes, rengolin

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71216