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

[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