[clang] [Clang] [C++26] Expansion Statements (Part 1: AST) (PR #169680)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I think I'm ok with this one.  There's a lot of stuff in this patch, but I did as good of a review as I could, and it seems reasonable. 

https://github.com/llvm/llvm-project/pull/169680