[Ada] Fix spurious errors on ghost code in generics

[Pierre-Marie de Rodat via Gcc-patches]

After the recent fix for detecting illegal use of ghost entities in
code, spurious errors could be raised on generic code with ghost, due to
wrong setting of the ghost flags on copied entities from the generic to
the instantiation.

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

gcc/ada/

        * atree.adb (New_Copy): Reset flags related to ghost entities
        before marking the new node.
diff --git a/gcc/ada/atree.adb b/gcc/ada/atree.adb
--- a/gcc/ada/atree.adb
+++ b/gcc/ada/atree.adb
@@ -1775,6 +1775,11 @@ package body Atree is
 
          --  Mark the copy as Ghost depending on the current Ghost region
 
+         if Nkind (New_Id) in N_Entity then
+            Set_Is_Checked_Ghost_Entity (New_Id, False);
+            Set_Is_Ignored_Ghost_Entity (New_Id, False);
+         end if;
+
          Mark_New_Ghost_Node (New_Id);
 
          New_Node_Debugging_Output (New_Id);