ada: Add diagnostic entry consistency checks

ada: Add diagnostic entry consistency checks

Verify that every diagnostic that has a switch also has the same
diagnostic marked as one of the diagnostics for that same switch.

Additionally verify that for every diagnostic marked for a switch
these diagnostics have the same switch marked as its switch.

gcc/ada/ChangeLog:

	* errid.adb (Check_Diagnostic_To_Switch_Consistency): New subprogram
	for checking the consistency of diagnostics.
	(Check_Switch_To_Diagnostic_Consistency) New subprogram for checking
	the consistency of switches.
	(Add_All_Diagnostic_Rules): Check diagnostic consitency.
	(Add_All_Switch_Rules): Check switch consitency.