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

[Alex Bradbury via cfe-commits]

asb wrote:

It looks like I wrote the original logic here way way back. I've scanned my memory banks as well as any notes I have. I don't think I had any particular perception implementing it in the way it is today was necessary for compliance with the ABI. I think it just made sense to me at the time when following along the ABI. Avoiding unnecessary extensions as this PR does seems worth doing to me.

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