[Ada] Accept Structural in aspect Subprogram_Variant and pragma Loop_Variant

[Ada] Accept Structural in aspect Subprogram_Variant and pragma Loop_Variant

Add a new form of variants to ensure termination of loops or recursive
subprograms. Structural variants correspond to objects which designate a
part of the data-structure they used to designate in the previous loop
iteration or recursive call. They only imply termination if the
data-structure is acyclic, which is the case in SPARK but not in Ada in
general. The fact that these variants are correct is only verified
formally by the proof tool and not by the compiler or dynamically at
execution like other forms of variants.

gcc/ada/

	* snames.ads-tmpl: Add "Structural" as a name.
	* sem_prag.adb: (Analyze_Pragma): Accept modifier "Structural"
	in pragmas Loop_Variant and Subprogram_Variant. Check that items
	associated to Structural occur alone in the pragma associations.
	(Analyze_Subprogram_Variant_In_Decl_Part): Idem.
	* exp_prag.adb (Expand_Pragma_Loop_Variant): Discard structural
	variants.
	(Expand_Pragma_Subprogram_Variant): Idem.

gcc/testsuite/

	* gnat.dg/loopvar.adb: Update expected error message.