[coroutines] Include the implicit object parameter type when looking up coroutine_traits for member functions.

This patch was originally from Toby Allsopp, but I hijacked it and
fixed it up with his permission.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307513 91177308-0d34-0410-b5e6-96231b3b80d8