[Ada] Simplify call to overloaded Earlier_In_Extended_Unit

[Pierre-Marie de Rodat via Gcc-patches]

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.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_warn.adb (Check_Unset_Reference): Use variant of
        Earlier_In_Extended_Unit that calls Sloc internally.
diff --git a/gcc/ada/sem_warn.adb b/gcc/ada/sem_warn.adb
--- a/gcc/ada/sem_warn.adb
+++ b/gcc/ada/sem_warn.adb
@@ -1915,7 +1915,7 @@ package body Sem_Warn is
                  and then (No (Unset_Reference (E))
                             or else
                               Earlier_In_Extended_Unit
-                                (Sloc (N), Sloc (Unset_Reference (E))))
+                                (N, Unset_Reference (E)))
                  and then not Has_Pragma_Unmodified_Check_Spec (E)
                  and then not Warnings_Off_Check_Spec (E)
                  and then not Has_Junk_Name (E)