ada: Inheritance of pragma/aspect unchecked_union

ada: Inheritance of pragma/aspect unchecked_union

Derived types inherit pragma/aspect uncheched union.

gcc/ada/ChangeLog:

	* sem_ch3.adb (Build_Derived_Record_Type): Record type derivations
	inherit Is_Unchecked_Union and Has_Unchecked_Union flags.
	(Inherit_Component): Add discriminals to the associations list.
	* exp_ch3.adb (Build_Record_Init_Proc): Derivations of Unchecked_Union
	types don't need an initialization procedure; they reuse the init proc
	of their parent type.