This is that diff I was aiming for. When transitioning code from
coroutines-ts to c++20, it can be useful to add a using declaration to
std::experimental pointing to std::coroutine_traits. This permits
that use by checking whether lookup in std::experimentl finds a
different decl to lookup in std. You still get a warning about
std::experimental::coroutine_traits being a thing, just not an error.
Reviewed By: ChuanqiXu
Differential Revision: https://reviews.llvm.org/D115943
Nathan Sidwell