[Coroutines] checkAsyncFuncPointer - use cast<> instead of dyn_cast<> for...

[Coroutines] checkAsyncFuncPointer - use cast<> instead of dyn_cast<> for dereferenced pointer. NFCI.

We're immediately dereferencing the casted pointer, so use cast<> which will assert instead of dyn_cast<> which can return null.

Fixes static analyzer warning.