[COFF, ARM64] Fix ABI implementation of struct returns

Summary:
Refer the ABI doc at: https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=vs-2019#return-values

Related clang patch: D60349

Reviewers: rnk, efriedma, TomTan, ssijaric

Reviewed By: rnk, efriedma

Subscribers: mstorsjo, javed.absar, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60348

llvm-svn: 359934