[Ada] Elaboration order v4.0 activation

[Pierre-Marie de Rodat]

This patch enables the elaboration order v4.0 as the default elaboration
order in GNATbind. The previous v3.0 elaboration order is now referred
to as the "legacy elaboration order mechanism" and is available using
binder switch -H.

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

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

gcc/ada/

	* bindo.adb: Remove with and use clauses for Debug.  Add with
	and use clauses for Opt.
	(Find_Elaboration_Order): Enable the v4.0 elaboration order. The
	v3.0 mechanism is now available under binder switch -H.
	* bindusg.adb (Display): Enable switch -H.
	* debug.adb: Free compiler switch -gnatd_G.  Free binder switch
	-d_N.
	* sem_elab.adb: Update the section on switches to remove
	-gnatd_G.
	(Invocation_Graph_Recording_OK): The invocation graph is now
	unconditionally recorded in ALI files.
	* switch-b.adb (Scan_Binder_Switches): Scan switch -H.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Update the documentation on compiler switches related to
	elaboration.  Update the documentation on binder switches to
	include switch -H.
	* doc/gnat_ugn/elaboration_order_handling_in_gnat.rst: Update
	the documentation on elaboration order handling in GNAT.
	* gnat_ugn.texi: Regenerate.
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