ada: Change "missing overriding indicator" message from error to warning

ada: Change "missing overriding indicator" message from error to warning

There is no RM rule requiring an overriding indicator in the case where
this message is generated; such a rule was discussed many years ago in an
AI, but that AI was never approved. So generate a warning message instead
of an error message. And don't even do that if we are in an instance (warning
a user they should change the source of an instance seems unlikely to be
helpful, at least in this case).

gcc/ada/

	* sem_disp.adb (Check_Dispatching_Operation): When calling
	Error_Msg_NE to generate a "missing overriding indicator" message,
	generate a warning message instead of an error message (and update
	comment to describe this new behavior).