[Ada] Support for new aspect Subprogram_Variant on recursive subprograms

[Ada] Support for new aspect Subprogram_Variant on recursive subprograms

gcc/ada/

	* aspects.ads: Introduce Subprogram_Variant aspect with the
	following properties: GNAT-specific, with mandatory expression,
	not a representation aspect, never delayed.
	* contracts.adb (Expand_Subprogram_Contract): Mention new aspect
	in the comment.
	(Add_Contract_Item): Support addition of pragma
	Subprogram_Variant to N_Contract node.
	(Analyze_Entry_Or_Subprogram_Contract): Mention new aspect in
	the comment; add pragma Subprogram_Variant to N_Contract node.
	(Build_Postconditions_Procedure): Adapt call to
	Insert_Before_First_Source_Declaration, which is now reused in
	expansion of new aspect.
	(Process_Contract_Cases_For): Also process Subprogram_Variant,
	which is stored in N_Contract node together with Contract_Cases.
	* contracts.ads (Analyze_Entry_Or_Subprogram_Contract): Mention
	new aspect in the comment.
	(Analyze_Entry_Or_Subprogram_Body_Contract): Likewise.
	* einfo.adb (Get_Pragma): Support retrieval of new pragma.
	* einfo.ads (Get_Pragma): Likewise.
	* exp_ch6.adb (Check_Subprogram_Variant): New routine for
	emitting call to check Subprogram_Variant expressions at run
	time.
	(Expand_Call_Helper): Check Subprogram_Variant expressions at
	recursive calls.
	* exp_prag.adb (Make_Op): Moved from expansion of pragma
	Loop_Variant to Exp_Util, so it is now reused for expansion of
	pragma Subprogram_Variant.
	(Process_Variant): Adapt call to Make_Op after moving it to
	Exp_Util.
	(Expand_Pragma_Subprogram_Variant): New routine.
	* exp_prag.ads (Expand_Pragma_Subprogram_Variant): Likewise.
	* exp_util.adb (Make_Variant_Comparison): Moved from Exp_Prag
	(see above).
	* exp_util.ads (Make_Variant_Comparison): Likewise.
	* inline.adb (Remove_Aspects_And_Pragmas): Handle aspect/pragma
	Subprogram_Variant just like similar contracts.
	* par-prag.adb (Prag): Likewise.
	* sem.adb (Insert_Before_First_Source_Declaration): Moved from
	Contracts (see above).
	* sem.ads (Insert_Before_First_Source_Declaration): Likewise.
	* sem_ch12.adb: Mention new aspect in the comment about
	"Implementation of Generic Contracts", just like similar aspects
	are mentioned there.
	* sem_ch13.adb (Insert_Pragma): Mention new aspect in the
	comment, because this routine is now used for Subprogram_Variant
	just like for other similar aspects.
	(Analyze_Aspect_Specifications): Mention new aspect in comments;
	it is handled just like aspect Contract_Cases.
	(Check_Aspect_At_Freeze_Point): Do not expect aspect
	Subprogram_Variant just like we don't expect aspect
	Contract_Cases.
	* sem_prag.adb (Ensure_Aggregate_Form): Now also used for pragma
	Subprogram_Variant, so update comment.
	(Analyze_Pragma): Add initial checks for pragma
	Subprogram_Variant.
	(Analyze_Subprogram_Variant_In_Decl_Part): New routine with
	secondary checks on the new pragma.
	(Sig_Flags): Handle references within pragma Subprogram_Variant
	expression just like references in similar pragma
	Contract_Cases.
	(Is_Valid_Assertion_Kind): Handle Subprogram_Variant just like
	other similar contracts.
	* sem_prag.ads (Analyze_Subprogram_Variant_In_Decl_Part): New
	routine.
	* sem_res.adb (Same_Or_Aliased_Subprograms): Moved to Sem_Util,
	so it can be reused for detection of recursive calls where
	Subprogram_Variant needs to be verified.
	* sem_util.adb (Is_Subprogram_Contract_Annotation): Handle new
	Subprogram_Variant annotation just like other similar
	annotations.
	(Same_Or_Aliased_Subprograms): Moved from Sem_Res (see above).
	* sem_util.ads (Is_Subprogram_Contract_Annotation): Mention new
	aspect in the comment.
	(Same_Or_Aliased_Subprograms): Moved from Sem_Res (see above).
	* sinfo.ads (N_Contract): Document handling of
	Subprogram_Variant.
	* snames.ads-tmpl: Add name for the internally generated
	procedure with checks for Subprogram_Variant expression, name
	for the new aspect and new pragma corresponding to aspect
	Subprogram_Variant.