[Ada] Implement AI12-0269 No_Return for functions

[Ada] Implement AI12-0269 No_Return for functions

2020-06-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (No_Return): Document it for all subprograms.
	* einfo.adb (Set_No_Return): Adjust assertion accordingly.
	* sem_ch3.adb (Check_Abstract_Overriding): Implement the
	check prescribed by RM 6.5.1(6/2) here instead of...
	(Derive_Subprogram): Adjust comment accordingly.
	* sem_disp.adb (Override_Dispatching_Operation): ...here.
	Remove superfluous return statement.
	* sem_ch6.adb (Check_No_Return_Expression): New procedure.
	(Analyze_Function_Return): Call it to implement the check
	prescribed by AI12-0269 for simple return statements of
	No_Return functions, and also checks extended statements.
	(Analyze_Return_Statement): Only give an error on a return
	statement in No_Return procedures.  Use idiomatic form.
	* sem_ch8.adb (Analyze_Subprogram_Renaming): Adjust error
	message for No_Return renaming subprogram.
	* sem_prag.adb (Analyze_Pragma) <Pragma_No_Return>: Accept
	it on functions and generic functions in Ada 2020.