[coro.async] Make sure we process async coroutines

Because we were not looking for the llvm.coro.id.async intrinsic in the
early coro pass which triggers follow-up passes we relied on the
llvm.coro.end intrinsic being present. This might not be the case in
functions that end in unreachable code.

Differential Revision: https://reviews.llvm.org/D95144