[Ada] Warn on suspicious Subprogram'Access

[Arnaud Charlet <charlet at adacore dot com>]

This patch adds a warning on certain occurrences of Subprogram'Access that
could cause an access-before-elaboration error.

The following example should give a warning when compiled with the -gnatw.f
switch:

% gcc -c -gnatw.f -gnatwe elab_acc.adb
elab_acc.ads:4:31: warning: Access attribute of "F" before body seen
elab_acc.ads:4:31: warning: possible Program_Error on later references
%

package Elab_Acc is
   function F return Integer;
   type Funcy is access function return Integer;
   F_Ptr : constant Funcy := F'Access;
   --  Calls to F_Ptr.all before F's body can cause an
   --  access-before-elaboration error.

end Elab_Acc;

package body Elab_Acc is

   function F return Integer is
   begin
      return 123;
   end F;

end Elab_Acc;

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

2015-01-07  Bob Duff  <duff@adacore.com>

	* sem_elab.adb (Check_Internal_Call_Continue): Give a warning
	for P'Access, where P is a subprogram in the same package as
	the P'Access, and the P'Access is evaluated at elaboration
	time, and occurs before the body of P. For example, "X : T :=
	P'Access;" would allow a subsequent call to X.all to be an
	access-before-elaboration error; hence the warning. This warning
	is enabled by the -gnatw.f switch.
	* opt.ads (Warn_On_Elab_Access): New flag for warning switch.
	* warnsw.adb (Set_Dot_Warning_Switch): Set Warn_On_Elab_Access.
	* gnat_ugn.texi: Document the new warning.

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