[Ada] Fix -gnatw.f warnings not having the right insertion characters

[Pierre-Marie de Rodat via Gcc-patches]

This caused these warnings not to be tagged with the switch causing
them, which is an issue for tools ingesting GNAT's output.

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

gcc/ada/

        * sem_elab.adb (Process_Conditional_ABE_Access_Taken): Add '.f'
        insertion characters.
diff --git a/gcc/ada/sem_elab.adb b/gcc/ada/sem_elab.adb
--- a/gcc/ada/sem_elab.adb
+++ b/gcc/ada/sem_elab.adb
@@ -4958,7 +4958,7 @@ package body Sem_Elab is
          then
             Error_Msg_Name_1 := Attribute_Name (Attr);
             Error_Msg_NE
-              ("??% attribute of & before body seen", Attr, Subp_Id);
+              ("?.f?% attribute of & before body seen", Attr, Subp_Id);
             Error_Msg_N ("\possible Program_Error on later references", Attr);
 
             Output_Active_Scenarios (Attr, New_In_State);