ada: Cleanups in handling of aggregates
Assorted cleanups related to recent fixes of aggregate handling for
GNATprove; semantics is unaffected.
gcc/ada/
* sem_aggr.adb
(Resolve_Record_Aggregate): Remove useless assignment.
* sem_aux.adb
(Has_Variant_Part): Remove useless guard; this routine is only called
on type entities (and now will crash in other cases).
* sem_ch3.adb
(Create_Constrained_Components): Only assign Assoc_List when necessary;
tune whitespace.
(Is_Variant_Record): Refactor repeated calls to Parent.
* sem_util.adb
(Gather_Components): Assert that discriminant association has just one
choice in component_association; refactor repeated calls to Next.
* sem_util.ads
(Gather_Components): Tune whitespace in comment.