[clang] [Clang][LLVM][Coroutines] Prevent __coro_gro from outliving __promise (PR #66706)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

Thanks for the clarifications

> By forcing the GRO not living on the coroutine frames, it shouldn't be a problem if the lifetime of `__coro_gro` outlives the `__promise`. The only limit is that the initialization of `__coro_gro` should be in the lifetime of `__promise`.

Applied your suggestion based on #49843, and edited description + title.

https://github.com/llvm/llvm-project/pull/66706