[Ada] Fixes in the use of spans for error locations

[Ada] Fixes in the use of spans for error locations

gcc/ada/

	* errout.adb (Error_Msg_NEL): Extract span from node.
	(First_And_Last_Nodes): Use spans for subtype indications and
	attribute definition clauses.
	(Write_Source_Code_Lines): Fix for tabulation characters. Change
	output for large spans to skip intermediate lines.
	* sem_case.adb (Check_Choice_Set): Report duplicate choice on
	the Original_Node for the case.
	(Generic_Check_Choices): Set the Original_Node for the rewritten
	case, so that the subtree used in spans has the correct
	locations.