[Ada] Crash on if_expression used as index of discriminant-dependent array

[Ada] Crash on if_expression used as index of discriminant-dependent array

gcc/ada/

	* sem_res.adb (Resolve_If_Expression): If the context of the
	expression is an indexed_component, resolve the expression and
	its dependent_expressions with the base type of the index, to
	ensure that an index check is generated when resolving the
	enclosing indexxed_component, and avoid an improper use of
	discriminants out of scope, when the index type is
	discriminant-dependent.