[Ada] Further fix non-stored discriminant in aggregate for GNATprove

[Ada] Further fix non-stored discriminant in aggregate for GNATprove

GNATprove expects discriminants appearing in aggregates and their types
to be resolved to stored discriminants.  This extends the machinery that
makes sure this is the case for default initialization expressions so as
to also handle component associations in these expressions.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_aggr.adb (Rewrite_Bound): Be prepared for discriminals
	too.
	(Rewrite_Range;): Minor tweak.
	(Resolve_Record_Aggregate): For a component with default
	initialization whose expression is an array aggregate, also
	rewrite the bounds of the component associations, if any.

From-SVN: r273679