[Ada] Allow uses of range utility routines on private types

[Ada] Allow uses of range utility routines on private types

Frontend only calls Is_Null_Range and Not_Null_Range routines on full views
of types, but backends (for example GNATprove) might call them also on
private types. This patch adapts those routines to transparently retrieve
the full type when called on a private type.

No frontend test, because only external backends are affected.

2018-01-11  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_eval.adb (Is_Null_Range): Retrieve the full view when called on a
	private (sub)type; refactor to avoid early return statement.
	(Not_Null_Range): Same as above.

From-SVN: r256522