[clang] Diagnose use of VLAs in a coroutine (PR #70341)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> LGTM but it is missing a release note :)

it may be fine to not mention it clearly since VLAs in coroutines never got compiled.. user might meet backend crashes..

https://github.com/llvm/llvm-project/pull/70341