[flang] Emit error about missing interface when needed

[klausler]

Summary

When a procedure pointer with no interface is called by a
function reference, complain about the lack.

[PeteSteinfeld]

All builds, tests, and looks goodl.

flang/lib/Semantics/expression.cpp

             DEREF(result.GetTypeAndShape()).type(),
             ProcedureRef{std::move(proc), std::move(arguments)});
       }
+    } else {
+      Say("Function result characteristics are not known"_err_en_US);
     }
   }
   return std::nullopt;

flang/test/Semantics/resolve09.f90

   !ERROR: Cannot call function 'f' like a subroutine
   call f
   !ERROR: Cannot call subroutine 's' like a function
+  !ERROR: Function result characteristics are not known
   i = s()
 contains
   function f()