[clang] [Clang] [C++26] Implement P1306R5 Expansion Statements (PR #165195)

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Sirraide wrote:

> I have to agree w/ @erichkeane this is going to be a challenge to review properly.

Yeah, I’ve been thinking about this over the weekend, and I have a plan to split this into a stack of like 8 prs and then merge everything together once we’re done reviewing all of them (similarly to the big NNS refactor that happened not long ago), but I’ve been a bit under the weather this week so not sure how soon I’ll be able to get to that...

https://github.com/llvm/llvm-project/pull/165195