[clang] [Clang][Parser] Make 'T...[N]' within a function parameter a valid pack expansion prior to C++2c (PR #116332)

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cor3ntin wrote:

I did think about this more, and given we made the feature an extension we should not have 2 different meaning in different contexts, so I think improving the diagnostic is really the only thing we should consider here

https://github.com/llvm/llvm-project/pull/116332