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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

byval changes the calling convention: instead of passing a pointer, it copies the pointed-to value into the argument list.

It's not clear to me why msan cares about whether a pointer is an implicit or explicit value... but if it does, we'd want to use an attribute that doesn't have an existing ABI impact.

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