[COFF, ARM64] Decide when to mark struct returns as SRet

Summary:
Refer the MS ARM64 ABI Convention for the behavior for struct returns:
https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions#return-values

Reviewers: mstorsjo, compnerd, rnk, javed.absar, yinma, efriedma

Reviewed By: rnk, efriedma

Subscribers: haripul, TomTan, yinma, efriedma, kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D49464

llvm-svn: 338050