[Ada] Restore Snames.Name_SPARK as it used in the GNATprove backend

[Pierre-Marie de Rodat]

The Name_SPARK constant is no longer used in GNAT to implement the
SPARK_05 restriction, but is still used in GNATprove to detect whether
the SPARK.Heap unit has been explicitly WITH-ed.

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

2020-06-08  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * snames.ads-tmpl (Name_SPARK): Restore after being deleted.
--- gcc/ada/snames.ads-tmpl
+++ gcc/ada/snames.ads-tmpl
@@ -835,6 +835,7 @@ package Snames is
    Name_Section                        : constant Name_Id := N + $;
    Name_Semaphore                      : constant Name_Id := N + $;
    Name_Simple_Barriers                : constant Name_Id := N + $;
+   Name_SPARK                          : constant Name_Id := N + $;
    Name_SPARK_05                       : constant Name_Id := N + $;
    Name_Spec_File_Name                 : constant Name_Id := N + $;
    Name_State                          : constant Name_Id := N + $;