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

[via cfe-commits]

Sirraide wrote:

> I think the changes are fine now, but we should add one more test case:
> 
> ```c++
> // NO warning here. This could be a regular function returning a `Coro` object.
> [[noreturn]] Coro test2();
> ```
> 
> Putting the check in `CheckCompletedCoroutineBody()` means that this should already work properly, but let’s add a test just to be sure.

Admittedly, declaring *any* non-void function `noreturn` is semantically questionable imo, but that’s not really relevant for this pr.

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