[Ada] Enable delta_aggregate only in Ada 2020 mode

[Ada] Enable delta_aggregate only in Ada 2020 mode

If the delta_aggregate feature is used, the -gnat2020 or -gnatX switch
must be given.

The following test must get an error if neither
-gnat2020 nor -gnatX is specified:

gcc -c delta_aggregate.ads -gnat2012

delta_aggregate.ads:4:18: delta_aggregate is an Ada 202x feature
delta_aggregate.ads:4:18: compile with -gnatX

package Delta_Aggregate is

   X : String := "Hello";
   Y : String := (X with delta 1 => 'h');

end Delta_Aggregate;

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* par-ch4.adb: Minor wording change in error messages.
	* sem_aggr.adb (Resolve_Delta_Aggregate): Emit an error for Ada
	versions prior to Ada 2020.

From-SVN: r274729