[Ada] Check declare and qualified expressions for unset references

[Ada] Check declare and qualified expressions for unset references

Detection of references to unset (uninitialized) objects requires calls
to Check_Unset_Reference on every subexpression of a composite statement
and expression. For declare and qualified expressions this was done only
when they occurred within another composite statement/expression.

gcc/ada/

	* sem_res.adb (Resolve_Declare_Expression): Check expression for
	references to unset objects.
	(Resolve_Qualified_Expression): Likewise.
	* sem_warn.adb (Check_Unset_Reference): Remove handling of
	declare and qualified expressions; clarify comment for type
	conversions.