[Ada] Spurious 'W' ALI line due to implicit with clause

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch "fixes" an issue where an implicit with clause generated to emulate
an implicit Elaborate[_All] pragma appears on a 'W' line in the ALI file. As a
result, the 'W' line may introduce a spurious build dependency in GPRbuild.

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

--  func.ads

function Func return Boolean;

--  func.adb

function Func return Boolean is begin return True; end Func;

--  gen.ads

generic
package Gen is
   procedure Force_Body;
end Gen;

--  gen.adb

with Func;

package body Gen is
   Val : constant Boolean := Func;

   procedure Force_Body is begin null; end Force_Body;
end Gen;

--  pack.ads

with Gen;

package Pack is
   package Inst is new Gen;
end Pack;

--  main.adb

with Pack;

procedure Main is begin null; end Main;

----------------------------
-- Compilation and output --
----------------------------

$ gnatmake -q main.adb
$ grep -c "Z func" pack.ali
1

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

2017-12-15  Hristian Kirtchev  <kirtchev@adacore.com>

	* sem_elab.adb (Ensure_Prior_Elaboration_Static): Mark the generated
	with clause as being implicit for an instantiation in order to
	circumvent an issue with 'W' and 'Z' line encodings in ALI files.

Attachment: difs
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