[Ada] Fix indirect calls to imported subprograms within generic

[Pierre-Marie de Rodat <derodat@adacore.com>]

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Our trampoline avoidance scheme based on subprogram descriptors
should always be disconnected for subprograms with foreign convention.
For access to subprogram subtypes, the Set_Convention front-end
procedure is expected to take care of this, but it currently only
does so for subtypes not within a generic instance.

This causes wrong code to be generated on many targets for the
testcase below, where the indirect call from Trigger is performed
through a (non existent) descriptor, resulting in SEGV in most
circumstances.

/* cfoo.c */
void c_foo ()
{
 printf ("hello there from C");
}

-- blob.ads
generic
package Blob is
  procedure Initialize;
  procedure Trigger;
end;

-- blob.adb
package body Blob is

  type Service_Ptr is access procedure;
  pragma Convention (C, Service_Ptr);

  Hook : Service_Ptr;

  procedure C_Foo;
  pragma Import (C, C_Foo, "c_foo");

  procedure Initialize is
  begin
     Hook := C_Foo'Access;
  end;

  procedure Trigger is
  begin
     Hook.all;
  end;
end;

The change proposed here just arranges for the bit clearing
to happen systematically in Set_Convention, which fixes the case
at hand and just confirms the bit value in case the entity was
processed previously.

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

2017-12-05  Olivier Hainque  <hainque@adacore.com>

	* sem_util.adb (Set_Convention): Always clear Can_Use_Internal_Rep
	on access to subprogram types with foreign convention.


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Index: sem_util.adb
===================================================================
--- sem_util.adb	(revision 255412)
+++ sem_util.adb	(working copy)
@@ -23090,17 +23090,7 @@
         and then Is_Access_Subprogram_Type (Base_Type (E))
         and then Has_Foreign_Convention (E)
       then
-
-         --  A pragma Convention in an instance may apply to the subtype
-         --  created for a formal, in which case we have already verified
-         --  that conventions of actual and formal match and there is nothing
-         --  to flag on the subtype.
-
-         if In_Instance then
-            null;
-         else
-            Set_Can_Use_Internal_Rep (E, False);
-         end if;
+         Set_Can_Use_Internal_Rep (E, False);
       end if;
 
       --  If E is an object, including a component, and the type of E is an