This patch introduces binder switch -d_S which prompts the various
phases of the elaboration order mechanism to output a short message
concerning their commencement and completion. The output is useful when
trying to determine whether a phase is stuck in an infinite loop.

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

--  main.adb

procedure Main is begin null; end Main;

----------------------------
-- Compilation and output --
----------------------------

$ gnatmake -q main.adb -bargs -d_S -d_V
elaborating units...
collecting units...
units collected.
constructing library graph...
validating library graph...
library graph validated.
library graph constructed.
constructing invocation graph...
validating invocation graph...
invocation graph validated.
invocation graph constructed.
augmenting library graph...
library graph augmented.
discovering components...
components discovered.
validating elaboration order...
elaboration order validated.
units elaborated.

2019-07-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* bindo.adb: Update the section of switches and debugging
	elaboration issues.
	* bindo.ads: Add type Elaboration_Phase.
	* bindo-augmentors.adb: Add use clause for
	Bindo.Writers.Phase_Writers.
	(Augment_Library_Graph): Signal the start and end of the
	aubmentation phase.
	* bindo-builders.adb: Add with and use clause for Bindo.Writers.
	Add use clause for Bindo.Writers.Phase_Writers.
	(Build_Invocation_Graph): Signal the start and end of the
	invocation graph construction phase.
	(Build_Library_Graph): Signal the start and end of the library
	graph construction phase.
	* bindo-diagnostics.adb: Add use clause for
	Bindo.Writers.Phase_Writers.
	(Diagnose_Cycle): Signal the start and end of the cycle
	diagnostic phase.
	* bindo-elaborators.adb: Add use clause for
	Bindo.Writers.Phase_Writers.
	(Elaborate_Units): Signal the start and end of the unit
	elaboration phase.
	* bindo-graphs.adb: Add use clause for
	Bindo.Writers.Phase_Writers.
	(Find_Components): Signal the start and end of the component
	discovery phase.
	(Find_Cycles): Signal the start and end of the cycle discovery
	phase.
	* bindo-units.adb: Add with and use clause for Bindo.Writers.
	Add use clause for Bindo.Writers.Phase_Writers.
	(Collect_Elaborable_Units): Signal the start and end of the unit
	collection phase.
	* bindo-validators.adb: Add with and use clause for
	Bindo.Writers.  Add use clause for Bindo.Writers.Phase_Writers.
	(Validate_Cycles, Validate_Elaboration_Order,
	Validate_Invocation_Graph, Validate_Library_Graph): Signal the
	start and end of the libray graph validation phase.
	* bindo-writers.ads, bindo-writers.adb: Add new nested package
	Phase_Writers.
	* debug.adb: Update the documentation of switch d_S.

From-SVN: r273379