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

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Sirraide 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),

I think we should just remove it (maybe make it an error that can be downgraded to a warning for now and then a hard error in the next release? not sure that’s even necessary tho), because, candidly, it just seems categorically useless (and every other compiler seems to agree considering that they don’t support it to begin with).

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