[Ada] Ada_2020: contracts for formal subprograms

[Ada] Ada_2020: contracts for formal subprograms

2020-06-04  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch12.adb (Build_Suprogram_Body_Wrapper,
	Build_Subprogram_Decl_Wrapper): New suprograms, to create the
	wrappers needed to implement contracts on formsl subprograms at
	the point of instantiation.
	(Build_Subprogram_Wrappers): New subprogram within
	Analyze_Associations, calls the above when the formal subprogram
	has contracts, and expansion is enabled.
	(Instantiate_Formal_Subprogram): If the actual is not an entity,
	such as a function attribute, or a synchronized operation,
	create a function with an internal name and call it within the
	wrapper.
	(Analyze_Generic_Formal_Part): Analyze contracts at the end of
	the list of formal declarations.
	* sem_prag.adb (Analyze_Pre_Post_Condtion): In Ada_2020 the
	aspect and corresponding pragma can appear on a formal
	subprogram declaration.
	(Find_Related_Declaration_Or_Body): Ditto.