[NFC] [Coroutines] Refactor implementation in checkFinalSuspendNoThrow

Now when we are checking if the expression `co_await
promise.final_suspend()` is not throw, we would check unconditionally
for its child expressions recursively. It takes unnecessary time. And
the compiler would complains if the implementation in final_suspend()
may throw even if the higher level function signature marked noexcept
already.

This fixes bug48453 too.