Fix bug in recursive_generator<T> when building under Clang.

The promise_type::get_return_object() method was returning a reference
to the promise object, expecting that reference to be passed to the
constructor of recursive_generator<T>.

However, under Clang the result of get_return_object() is assigned to
an 'auto' variable, which ends up taking a copy of the promise object
on the stack and passing that copy into the recursive_generator<T>
constructor.

Modified get_return_object() to return the already-constructed
recursive_generator<T> object.

include/cppcoro/recursive_generator.hpp

@@ -28,9 +28,12 @@ namespace cppcoro
 				, m_parentOrLeaf(this)
 			{}
 
-			promise_type& get_return_object() noexcept
+			promise_type(const promise_type&) = delete;
+			promise_type(promise_type&&) = delete;
+
+			auto get_return_object() noexcept
 			{
-				return *this;
+				return recursive_generator<T>{ *this };
 			}
 
 			std::experimental::suspend_always initial_suspend() noexcept