[Ada] Spurious error on Part_Of indicator

[Pierre-Marie de Rodat]

This patch modifies the verification of a missing Part_Of indicator to avoid
considering constants as visible state of a package instantiation because the
compiler cannot determine whether their values depend on variable input. This
diagnostic is left to GNATprove.

------------
-- Source --
------------

--  gnat.adc

pragma SPARK_Mode;

--  gen_pack.ads

generic
package Gen_Pack is
   Val : constant Integer := 123;
end Gen_Pack;

--  pack.ads

with Gen_Pack;

package Pack
  with Abstract_State => Pack_State
is
   procedure Force_Body;
private
   package Inst_1 is new Gen_Pack;                                   --  OK
   package Inst_2 is new Gen_Pack with Part_Of => Pack_State;        --  OK
end Pack;

--  pack.adb

package body Pack
  with Refined_State => (Pack_State => Inst_2.Val)
is
   procedure Force_Body is null;
end Pack;

-----------------
-- Compilation --
-----------------

$ gcc -c pack.adb

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

2018-07-17  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_prag.adb (Has_Visible_State): Do not consider constants as
	visible state because it is not possible to determine whether a
	constant depends on variable input.
	(Propagate_Part_Of): Add comment clarifying the behavior with respect
	to constant.
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