[Ada] Improve error message for Object_Size clause on dynamic array

This makes the compiler issue the same error:

    size clause not allowed for variable length type

for an Object_Size clause on a variable-sized type as for a Size clause,
for example on the following procedure:

procedure P (X, Y : Integer) is
   subtype Sub is String (X .. Y) with Object_Size => 64;
begin
   null;
end;

2019-08-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* freeze.adb (Freeze_Entity): Give the same error for an
	Object_Size clause on a variable-sized type as for a Size
	clause.

From-SVN: r274294

gcc/ada/ChangeLog

+2019-08-12  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* freeze.adb (Freeze_Entity): Give the same error for an
+	Object_Size clause on a variable-sized type as for a Size
+	clause.
+
 2019-08-12  Gary Dismukes  <dismukes@adacore.com>
 
 	* sem_prag.adb (Analyze_Pragma, Pragma_Suppress_Initialization):

gcc/ada/freeze.adb

@@ -6803,7 +6803,7 @@ package body Freeze is
          --  Do not allow a size clause for a type which does not have a size
          --  that is known at compile time
 
-         if Has_Size_Clause (E)
+         if (Has_Size_Clause (E) or else Has_Object_Size_Clause (E))
            and then not Size_Known_At_Compile_Time (E)
          then
             --  Suppress this message if errors posted on E, even if we are