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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

We basically have the following options here:

1. Use C++23 semantics in C++23 mode, with a warning.
2. Use C++2c semantics in C++23 mode, with a warning.
3. Reject the construct with a hard error in C++23 mode.

(2) and (3) are basically removing the pack expansion parse from the C++23 grammar.  Maybe we should ask the committee for a DR number to explicitly bless this approach.

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