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

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cor3ntin wrote:

> It was quite extensively discussed in https://github.com/llvm/llvm-project/pull/84184, but it wasn't deemed necessary to go through RFC process for a non-functional change that doesn't affect stable interfaces.

We had buy-in to split languages out and I think that has been successful (with painful churns).
Splitting out individual C++ features or even C from C++ is a different proposition all together because these things are naturally interweaved, and we quickly get diminishing returns (and increased churns), at the cost of more clumsy APIs
(ie things like `SemaAccess()`  _everywhere_ and thing harder to find or organized)

I think there are a lots of opportunities to remove some utilities objects out of Sema.h, move pragma handling etc which should be a bit more self contained before considering doing more surgery on C++ (and at some point we should pause and consider the impact of already done work)
There are a lot of architectures specific functions that we might be able to move, if we want to?




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