[Ada] Improve generated code for initialization of atomics

[Pierre-Marie de Rodat]

This patch makes the temp for initialization of an atomic variable be
constant.

No change in behavior; no test.

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

2019-08-20  Bob Duff  <d...@adacore.com>

gcc/ada/

        * freeze.adb (Is_Atomic_VFA_Aggregate): Make the temp for
        initialization of the atomic variable be constant. This is
        cleaner, and might improve efficiency.
--- gcc/ada/freeze.adb
+++ gcc/ada/freeze.adb
@@ -1771,6 +1771,7 @@ package body Freeze is
       New_N :=
         Make_Object_Declaration (Loc,
           Defining_Identifier => Temp,
+          Constant_Present    => True,
           Object_Definition   => New_Occurrence_Of (Typ, Loc),
           Expression          => Relocate_Node (N));
       Insert_Before (Par, New_N);