Rollup merge of #153398 - folkertdev:const-c-variadic-trailing-zst, r=RalfJung

fix ICE in `const_c_variadic` when passing ZSTs

fixes https://github.com/rust-lang/rust/issues/153351
r? RalfJung

There was a mismatch between the caller and callee ABI where the caller does not pass ZST arguments, but the callee does expect them. Because ZSTs don't implement `VaArgSafe` the program must already be invalid if this comes up.