[clang] [C23] Disable diagnostic on struct defn in prototype (PR #138516)

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cor3ntin wrote:

> This sorta thing is making me grumpy @ wg14 :D But this looks right to me.

Right, this is optimizing for something no one is ever going to do... at the same time I hope defining types in parameters
is not something people do at all, so either way, the impact is minimal, I think 



https://github.com/llvm/llvm-project/pull/138516