[Clang][Parser] Add a warning to ambiguous uses of T...[N] types (#116332)
`void f(T... [N])` is no longer treated as a function with a parameter
of pack expansion type after the implementation of the pack indexing
feature. This patch introduces a warning to clarify such cases while
maintaining it as a pack indexing type in all language modes.
Closes https://github.com/llvm/llvm-project/issues/115222