[Ada] Ada2020: AI12-0279 more dispatching points with aspect Yield

[Ada] Ada2020: AI12-0279 more dispatching points with aspect Yield

2020-06-17  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* aspects.ads (type Aspect_Id): Add Aspect_Yield as a Boolean
	aspect, and update the Is_Representation_Aspect, Aspect_Names,
	and Aspect_Delay arrays.
	* einfo.ads, einfo.adb (Has_Yield_Aspect, Yield_Aspect): New
	subprograms.
	* exp_ch6.adb (Add_Return, Expand_Non_Function_Return,
	Expand_Simple_Function_Return): Add calls to Yield.
	* exp_ch9.adb (Build_Accept_Body, Expand_N_Accept_Statement):
	Add calls to Yield.
	* rtsfind.ads (RE_Yield): Adding support to generate calls to
	the runtime service Ada.Dispatching.Yield
	* sem_ch13.adb (Analyze_Aspect_Yield): New subprogram.
	* sem_ch3.adb (Derive_Subprogram): Inherit attribute
	Has_Yield_Aspect.
	* sem_ch8.adb (Analyze_Subprogram_Renaming): Check consistency
	of Has_Yield in the actual subprogram of a generic
	instantiation.
	* sem_disp.adb (Check_Dispatching_Operation): Check that if the
	Yield aspect is specified for a dispatching subprogram that
	inherits the aspect, the specified value shall be confirming.
	* sem_prag.adb (Analyze_Pragma [Pragma_Implemented]): Check that
	the implementation kind By_Protected_Procedure cannot be applied
	to a procedure that has aspect Yield.