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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

This is a follow-up based on post-commit review feedback: https://github.com/llvm/llvm-project/pull/132939#issuecomment-2849999314 and is intended to match the behavior of GCC

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