[clang] [Clang] Warn against unused parameters in C++ coroutines with `-Wunused-parameters` (PR #70567)

[Yuxuan Chen via cfe-commits]

yuxuanchen1997 wrote:

@ChuanqiXu9 , I think I got it wrong last time. It really should be a trivial change. Let me prepare a new PR. 

https://github.com/llvm/llvm-project/pull/70567