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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The current spec language is:

> Empty structs or union arguments or return values are ignored by C compilers which support them as a non-standard extension. This is not the case for C++, which requires them to be sized types.

So empty structs in C are ignored, empty structs in C++ are treated as non-empty.

The "C++ struct containing an empty array" is sort of an edge case, since it's not actually legal C++, but the spec says the distinguishing factor is whether we're in C++ mode.  And gcc seems to do that, so let's do the same thing.  So instead of checking for `Size == 0`, check for `LangOpts.CPlusPlus`.

Also, please add a release note for this.

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