[coroutines] Fix checking for prvalue-ness of `await_suspend` return type

Summary:
@rsmith Does this correctly address the issues mentioned in https://reviews.llvm.org/D33625#inline-292971 ?



Reviewers: rsmith, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits, rsmith

Differential Revision: https://reviews.llvm.org/D33636

llvm-svn: 304373