ada: Fix crash on vector initialization

Initializing a vector using

 Vec : V.Vector := [Some_Type'(Some_Abstract_Type with F => 0)];

may crash the compiler. The expander marks the N_Extension_Aggregate for
delayed expansion which never happens and incorrectly ends up in gigi.

The delayed expansion is needed for nested aggregates, which the
original code is testing for, but container aggregates are handled
differently.

Such assignments to container aggregates are later transformed into
procedure calls to the procedures named in the Aggregate aspect
definition, for which the delayed expansion is not required/expected.

gcc/ada/

	* exp_aggr.adb (Convert_To_Assignments): Do not mark node for
	delayed expansion if parent type has the Aggregate aspect.
	* sem_util.adb (Is_Container_Aggregate): Move...
	* sem_util.ads (Is_Container_Aggregate): ... here and make it
	public.