[COFF, ARM64] Fix ABI implementation of struct returns

Summary:
Related llvm patch: D60348.
Patch co-authored by Sanjin Sijaric.

Reviewers: rnk, efriedma, TomTan, ssijaric, ostannard

Reviewed By: efriedma

Subscribers: dmajor, richard.townsend.arm, ostannard, javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60349

llvm-svn: 359932