[clang] [clang][CodeGen] Use byval for SystemZ indirect arguments (PR #66404)

[Ilya Leoshkevich via cfe-commits]

iii-i wrote:

I don't quite get why can't we use byval here. How is this different from x86_64? Both s390x and x86_64 ABIs require passing large structs via a pointer, why can x86_64 implement this using byval in LLVM and s390x can't? I agree that currently s390x backend does not handle it properly and the current version of this PR is not suitable for inclusion, but what is the conceptual problem here?

https://github.com/llvm/llvm-project/pull/66404