[clang] [Clang] Warn about `[[noreturn]]` on coroutines (PR #127623)

[via cfe-commits]

https://github.com/Sirraide commented:

I think the changes are fine now, but we should add one more:
```c++
// NO warning here. This could be a regular function returning a `Coro` object.
[[noreturn]] Coro test();
```
Putting the check in `CheckCompletedCoroutineBody()` means that this should already work properly, but let’s add a test just to be sure.

https://github.com/llvm/llvm-project/pull/127623