[Ada] Ada2020: AI12-0117 Restriction No_Tasks_Unassigned_To_CPU

[Ada] Ada2020: AI12-0117 Restriction No_Tasks_Unassigned_To_CPU

gcc/ada/

	* gnatbind.adb (Gnatbind): For No_Tasks_Unassigned_To_CPU, check
	that CPU has been set on the main subprogram.
	(Restriction_Could_Be_Set): Don't print
	No_Tasks_Unassigned_To_CPU if it would violate the
	above-mentioned rule. Up to now, all restrictions were checked
	by the compiler, with the binder just checking for consistency.
	But the compiler can't know which subprogram is the main, so
	it's impossible to check this one at compile time.
	* restrict.ads, restrict.adb: Misc refactoring. Change Warning
	to Warn, for consistency, since most already use Warn.
	(Set_Restriction): New convenience routine.
	* sem_ch13.adb (Attribute_CPU): Check
	No_Tasks_Unassigned_To_CPU.
	* sem_prag.adb (Pragma_CPU): Check No_Tasks_Unassigned_To_CPU.
	Misc refactoring.
	* tbuild.ads, tbuild.adb (Sel_Comp): New functions for building
	selected components.