[Ada] Fix indirect calls to imported subprograms within generic

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

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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