[Ada] Check if- and case-expressions for unset references

[Ada] Check if- and case-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. This was missing for if-expressions and incomplete for
case-expressions.

gcc/ada/

	* sem_res.adb (Resolve_Case_Expression): Check alternative
	expressions for references to unset objects.
	(Resolve_If_Expression): Check condition, then and else
	expressions for references to unset objects.