[Ada] Fix composability of return on the secondary stack
Having components that need to be returned on the secondary stack would
not always force a record type to be returned on the secondary stack
itself.
gcc/ada/
* sem_util.adb
(Returns_On_Secondary_Stack.Caller_Known_Size_Record): Directly
check the dependence on discriminants for the variant part, if
any, instead of calling the Is_Definite_Subtype predicate.