[clang] [Coroutines] Allow [[clang::coro_wrapper]] for class (PR #93268)

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snarkmaster wrote:

@ChuanqiXu9, I just ran across this PR -- and I immediately wondered if it could be used to improve stack-use-after-free checks for `folly::result` (the short-circuiting coro you saw on my recent PR). 

Like the classes you mention in the PR description here, `folly::result` has many reasonable "coro" and "coro wrapper" uses, and it would be onerous to annotate every wrapper usage as `coro_wrapper`.

What did you envision the effect of adding `coro_wrapper` to a class would be on lifetime checks? 

Can you walk me through your reasoning / expected usage?

https://github.com/llvm/llvm-project/pull/93268