[clang] [RISCV] Handle empty structs/unions passing in C++ (PR #97315)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> What about the case I have mentioned in https://godbolt.org/z/vdhGbvj6W ?

That test doesn't really prove anything useful.  The zeroing of a0 doesn't have any ABI significance: "struct s12" doesn't have any data in it, so the caller can't use the value in a0 for anything.  At most, it's a hint about how gcc's internal logic works.

https://github.com/llvm/llvm-project/pull/97315