ada: Support qualified error names from Standard

Typically nodes from Standard were not printed without the
fully qualified prefix names even when Error_Msg_Qual_Level was set.
However in some cases it makes sense to emphasize that the variable
is coming from Standard.

Add a new variable called Error_Msg_Qual_For_Standard to enable
qualified names for entities coming from Standard.

gcc/ada/ChangeLog:

	* err_vars.ads (Error_Msg_Qual_For_Standard): New Variable.
	* errout.ads (Error_Msg_Qual_For_Standard): Likewise.
	* errout.adb (Set_Qualification): Print all qualified names
	when Error_Msg_Qual_For_Standard is set.
	* sem_util.adb (Normalize_Actuals): Improve error message.