ada: Check validity using signedness from the type and not its base type

ada: Check validity using signedness from the type and not its base type

When attribute Valid is applied to a private type, we used the signedness of
its implementation base type which wrongly included negative values.

gcc/ada/ChangeLog:

	* exp_attr.adb (Expand_N_Attribute_Reference): When expanding attribute
	Valid, use signedness from the validated view, not from its base type.