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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

In any case:

- We absolutely can't use byval here.
- If you think we need a new attribute for this, please start a Discourse thread to get some feedback.  (I'm still not understanding what you're trying to fix here, but maybe additional perspectives would help bridge the gap.)

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