[clang] [Clang][P1061] Add stuctured binding packs (PR #121417)

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+// RUN: %clang_cc1 -fsyntax-only -std=c++26 %s -verify
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cor3ntin wrote:

Can we 
 - Test that `sizeof` (in the case where it's empty, of size 1, n, etc_) works fine
 -  Test that `auto [a, b...]` works for an array of size 1
 - Test that `auto [a, b..., c]` errors for an array of size 1
 - Test that it works fine with pack indexing

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p1061r10.html#pnum_5 Adding that verbatim would be a way to write some of these

Can we add this example?
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p1061r10.html#pnum_6

Can we add this example
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p1061r10.html#pnum_22

https://github.com/llvm/llvm-project/pull/121417