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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> gcc does ignore the empty array case for assigning argument registers in C++. Compare the two cases here https://godbolt.org/z/j4WP89rE8

Oh, you're right, I didn't actually do any testing myself, I was just assuming @svs-quic did the right test.

So I guess the previous size==0 check was actually correct.

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