[clang] [llvm] [Clang][Coroutines] Improve CoroElide with [[clang::coro_structured_concurrency]] attribute for C++ (PR #94693)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Then we can find that the frontend part is much more complex and harder than the middle end part.  So maybe we can introduce the middle end part first and introduce a not so powerful attribute but introducing a function and statement level attribute `must_elide` in the beginning.

https://github.com/llvm/llvm-project/pull/94693