[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 am not sure we want to keep supporting the old syntax, even in older language modes (notably, it isn't supported by other compilers and is at best missleading), but I'd like more people to chime in on that.

I'm all for improving diagnostics though.

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