Add [[nodiscard]] to friend declarations for clang 16 compatibility

Author: vinniefalco

include/boost/capy/async_result.hpp

@@ -163,6 +163,7 @@ class async_result
 
     template<class U, class DeferredOp>
         requires (!std::is_void_v<U>)
+    [[nodiscard]]
     friend async_result<U>
     make_async_result(DeferredOp&& op);
 
@@ -248,6 +249,7 @@ class async_result<void>
 
     template<class U, class DeferredOp>
         requires std::is_void_v<U>
+    [[nodiscard]]
     friend async_result<void>
     make_async_result(DeferredOp&& op);