[clang] [clang] Introduce `SemaCoroutine` (PR #92645)

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cor3ntin wrote:

Thanks! However, I am fairly against moving C and C++ features out of Sema, especially when the outcome is 50 Loc kess, at the cost of not knowing where to put or find things in the long term (as we add coroutines adjacent features, or code related to both coroutines and something else).

The previous sema changes made sense (despite the inevitable churn) - I am not sure this one does

https://github.com/llvm/llvm-project/pull/92645