[Ada] Ada_2020: Add aspect Aggregate to standard container units

[Ada] Ada_2020: Add aspect Aggregate to standard container units

gcc/ada/

	* exp_aggr.adb (Expand_N_Aggregate): A record aggregate requires
	a non-private type.
	* sem_ch13.adb (Valid_Assign_Indexed): New subprogram local to
	Resolve_Aspect_Aggregate, to handle the case when the
	corresponding name appearing in the aspect specification for an
	indexed aggregate is an overloaded operation.
	* libgnat/a-convec.ads, libgnat/a-convec.adb,
	libgnat/a-coinve.ads, libgnat/a-coinve.adb,
	libgnat/a-cobove.ads, libgnat/a-cobove.adb,
	libgnat/a-cdlili.ads, libgnat/a-cdlili.adb,
	libgnat/a-cbdlli.ads, libgnat/a-cbdlli.adb,
	libgnat/a-cohama.ads, libgnat/a-cihama.ads,
	libgnat/a-cbhama.ads, libgnat/a-cborma.ads,
	libgnat/a-ciorma.ads, libgnat/a-coorma.ads,
	libgnat/a-cihase.ads, libgnat/a-cohase.ads,
	libgnat/a-cbhase.ads, libgnat/a-cborse.ads,
	libgnat/a-ciorse.ads, libgnat/a-coorse.ads: Add Ada_2020 aspect
	Aggregate to types declared in standard containers, as well as
	new subprograms where required.