[Ada] Simplify call to overloaded Earlier_In_Extended_Unit

[Ada] Simplify call to overloaded Earlier_In_Extended_Unit

We have two variants of Earlier_In_Extended_Unit that take either
Node_Id or Source_Ptr values. The caller can simply use another variant
and not explicitly convert parameters.

Code cleanup; semantics is unaffected.

gcc/ada/

	* sem_warn.adb (Check_Unset_Reference): Use variant of
	Earlier_In_Extended_Unit that calls Sloc internally.