[clang] Allow using std::coroutine_traits in std::experimental

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