[Ada] Improve generated code for initialization of atomics

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

No change in behavior; no test.

2019-08-20  Bob Duff  <duff@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.

From-SVN: r274728

gcc/ada/ChangeLog

+2019-08-20  Bob Duff  <duff@adacore.com>
+
+	* freeze.adb (Is_Atomic_VFA_Aggregate): Make the temp for
+	initialization of the atomic variable be constant. This is
+	cleaner, and might improve efficiency.
+
 2019-08-20  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* repinfo.adb (List_Record_Info): In -gnatR4 mode, set the

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);