[Ada] Implement AI12-0187 (Stable properties of abstract data types)

[Ada] Implement AI12-0187 (Stable properties of abstract data types)

gcc/ada/

	* snames.ads-tmpl: Define new Name_Stable_Properties Name_Id
	value.
	* aspects.ads, aspects.adb: Add new Aspect_Stable_Properties
	enumeration literal to Aspect_Id type. Add Class_Present
	parameter to Find_Aspect and related
	functions (Find_Value_Of_Aspect and Has_Aspect).
	* sem_util.adb (Has_Nontrivial_Precondition): Fix
	previously-latent bug uncovered by adding Class_Present
	parameter to Aspect.Find_Aspect. The code was wrong before, but
	with the change the bug was more likely to make a user-visible
	difference.
	* sem_ch6.adb (Analyze_Operator_Symbol): If a string literal
	like "abs" or "-" occurs in a Stable_Properties aspect
	specification, then it is to be interpreted as an operator
	symbol and not as a string literal.
	* sem_ch13.ads: Export new Parse_Aspect_Stable_Properties
	function, analogous to the existing Parse_Aspect_Aggregate
	exported procedure.
	* sem_ch13.adb: (Parse_Aspect_Stable_Properties): New function;
	analogous to existing Parse_Aspect_Aggregate.
	(Validate_Aspect_Stable_Properties): New procedure; analogous to
	existing Validate_Aspect_Aggregate. Called from the same case
	statement (casing on an Aspect_Id value) where
	Validate_Aspect_Aggregate is called.
	(Resolve_Aspect_Stable_Properties): New procedure; analogous to
	existing Resolve_Aspect_Aggregate. Called from the same two case
	statements (each casing on an Aspect_Id value) where
	Resolve_Aspect_Aggregate is called.
	(Analyze_Aspect_Specifications): Set Has_Delayed_Aspects and
	Is_Delayed_Aspect attributes for Aspect_Stable_Properties aspect
	specifications.
	(Check_Aspect_At_End_Of_Declarations): The syntactic
	"expression" for a Stable_Properties aspect specification is not
	semantically an expression; it doesn't have a type. Thus, force
	T to be empty in this case.
	* contracts.adb (Expand_Subprogram_Contract): Add call to new
	local procedure,
	Expand_Subprogram_Contract.Add_Stable_Property_Contracts, which
	generates Postcondition pragmas corresponding to stable property
	checks.