[clang] [Clang] Mark P1061 (Structured Bindings can introduce a Pack) as implemented (PR #127980)

[via cfe-commits]

cor3ntin wrote:

Note that there does not seem to be any appetite to backport that to 20 - despite limited risks - so we mark it as a Clang 21 feature

https://github.com/llvm/llvm-project/pull/127980