[Ada] Factor out code for deciding statically known Constrained attributes

[Ada] Factor out code for deciding statically known Constrained attributes

Create a separate routine in Exp_Util for deciding the value of the
Constrained attribute when it is statically known. This routine is used
in Exp_Attr and will be reused in the backend of GNATprove.

There is no impact on compilation and hence no test.

2019-09-18  Claire Dross  <dross@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Call routine from
	Exp_Util to know the value of the Constrained attribute in the
	static case.
	* exp_spark.adb (Expand_SPARK_N_Attribute_Reference): Make
	implicit dereferences inside the Constrained attribute explicit.
	* exp_util.ads, exp_util.adb
	(Attribute_Constrained_Static_Value): New routine to compute the
	value of a statically known reference to the Constrained
	attribute.

From-SVN: r275848