[clang] [RISCV] Disable rounding of aggregate return/arguments to iXLen. (PR #184736)

[Sam Elliott via cfe-commits]

lenary wrote:

I played with your godbolt example a bit more, and I see we sort-of cannot escape extend instructions if we mark struct args/rets as zeroext/signext. If we `zeroext` struct args, then we need to zero-extend to put values of signed type into the arguments. If we `signext` them, then we need to sign-extend to put values of unsigned types into arguments.



https://github.com/llvm/llvm-project/pull/184736