[Ada] Elaboration order v4.0 and generic instantiations

[Ada] Elaboration order v4.0 and generic instantiations

This patch updates the library graph augmentation mechanism of the
elaboration order v4.0 to emulate a particular behavior of the v3.0
scheme involving generic instantiations. If a unit without any
elaboration code instantiates a generic without any elaboration code,
the invocation edge from the instance to the generic body is not
considered for library graph augmentation.

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

--  gen_pack1.ads

generic
   type Element_Type is private;

package Gen_Pack1 is
   procedure Read;
end Gen_Pack1;

--  gen_pack1.adb

with Types1;

package body Gen_Pack1 is
   procedure Read is null;
end Gen_Pack1;

--  types1.ads

with Gen_Pack1;

package Types1 is
   procedure Read;

   package Optional_Numbers is new Gen_Pack1 (Positive);
end Types1;

--  main1.adb

with Types1;

procedure Main1 is
begin
   Types1.Optional_Numbers.Read;
end Main1;

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

$ gnatmake -q main1.adb

2019-07-09  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* bindo-augmentors.adb (Visit_Elaboration_Root): Do not start a
	DFS from an elaboration root whose corresponding unit lacks
	elaboration code. This behavior mimics that of the old
	elaboration order mechanism.
	* bindo-graphs.adb (Find_All_Cycles_Through_Vertex): Move the
	vertex tracing within the functional branches of the routine.
	This prevents spurious trace output.
	(Has_No_Elaboration_Code): New routine.
	(Trace_Cycle, Trace_Edge): Update the various Ids to use the
	"standard" trace format.
	* bindo-graphs.ads (Has_No_Elaboration_Code): New routine.
	* bindo-units.ads, bindo-units.adb (Has_No_Elaboration_Code):
	New routine.

From-SVN: r273288