[Ada] More precise handling of Size/Object_Size in GNATprove

[Ada] More precise handling of Size/Object_Size in GNATprove

GNATprove does a partial expansion which did not allow getting the
most precise value for attributes Size/Object_Size. Now fixed.

There is no impact on compilation.

2019-08-12  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* exp_attr.adb, exp_attr.ads (Expand_Size_Attribute): New
	procedure to share part of the attribute expansion with
	GNATprove mode.
	(Expand_N_Attribute_Reference): Extract part of the
	Size/Object_Size expansion in the new procedure
	Expand_Size_Attribute.
	* exp_spark.adb (Expand_SPARK_N_Attribute_Reference): Expand
	Size/Object_Size attributes using the new procedure
	Expand_Size_Attribute.

From-SVN: r274290