[Ada] Fix expansion of aggregate for discriminated limited extension

[Ada] Fix expansion of aggregate for discriminated limited extension

The presence of the discriminants prevents the values associated with the
components of the parent type from being put into the sub-aggregate built
for the _Parent component.

gcc/ada/

	* exp_aggr.adb (Expand_Record_Aggregate.Build_Back_End_Aggregate):
	Skip the discriminants at the start of the component list before
	looking for the components inherited from the parent in the case
	of a tagged extension.