[Ada] Prevent search for references in postconditions from going too far

[Pierre-Marie de Rodat via Gcc-patches]

Add a standard prevention against climbing the entire compilation unit.
Cleanup only; behaviour of the compiler is unaffected.

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

gcc/ada/

        * sem_warn.adb (Within_Postcondition): Guard against search
        going too far.
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
@@ -2007,6 +2007,11 @@ package body Sem_Warn is
                               then
                                  return True;
                               end if;
+
+                           --  Prevent the search from going too far
+
+                           elsif Is_Body_Or_Package_Declaration (Nod) then
+                              exit;
                            end if;
 
                            Nod := Parent (Nod);