[Ada] Flexible AST node structure

[Ada] Flexible AST node structure

gcc/ada/

	* atree.ads: Make Default_Node a constant.  Remove the
	modification of Comes_From_Source, and use a separate flag for
	that.  Change Sloc to 0; it always overwritten, and never left
	as the No_Location value.
	(Print_Statistics): Move to spec so we can call it from
	gnat1drv.
	(Num_Nodes): Rename to clarify that this is approximate.
	Correct comment: nodes and entities are never deleted, the count
	is never decremented, and this is not used by Xref.
	(Initialize): Correct comment: Error_List is not created here.
	Other minor naming and comment changes.
	* atree.adb (Extend_Node, New_Copy, New_Entity, New_Node):
	Streamline these. Simplify and improve efficiency.  Move code
	from Allocate_Initialize_Node to these, where it can be executed
	unconditionally.  Take advantage of automatic zeroing of the
	Nodes table.
	(Allocate_Initialize_Node): Remove this. It was an efficiency
	bottleneck, and somewhat complicated, because it was called from
	4 places, and had all sorts of conditionals to check where it
	was called from. Better to move most of that code to the call
	sites, where it can be executed (or not) unconditionally.
	(Allocate_New_Node): New procedure to partly replace
	Allocate_Initialize_Node (called from just 2 of those 4 places).
	(Comes_From_Source_Default): New flag written/read by
	Set_Comes_From_Source_Default/Get_Comes_From_Source_Default.
	This allows us to make Default_Node into a constant with
	all-zeros value.
	(Set_Paren_Count_Of_Copy): New procedure to avoid duplicated
	code.
	(Report): New procedure to encapsulate the call to the reporting
	procedure.
	(Atree_Private_Part): We now need a body for this package, to
	contain package body Nodes.
	(Approx_Num_Nodes_And_Entities): Was Num_Nodes.  For efficiency,
	compute the answer from Nodes.Last. That way we don't need to
	increment a counter on every node creation. Other minor naming
	and comment changes.
	* gnat1drv.adb: Call Atree.Print_Statistics if -gnatd.A switch
	was given.  Add comment documenting the new order dependency (we
	must process the command line before calling Atree.Initialize).
	* debug.adb: Document -gnatd.A.
	* einfo.adb, sinfo.adb: Remove useless Style_Checks pragmas.
	* nlists.ads (Allocate_List_Tables): Inline makes node creation
	a little faster.
	* nlists.adb (Initialize): Remove local constant E, which didn't
	seem to add clarity.
	* treepr.adb (Print_Init): Use renamed
	Approx_Num_Nodes_And_Entities function.
	* types.ads: Change the Low and High bounds as described above.
	* types.h: Change Low and High bounds to match types.ads.
	* sem_ch8.adb, namet.adb, namet.ads: Move the computation of
	Last_Name_Id from sem_ch8 to namet, and correct it to not assume
	Name_Ids are positive.
	* ali.adb, ali-util.adb, bindo-writers.adb, exp_dist.adb,
	fmap.adb, fname-uf.adb, osint.adb: Fix various hash functions to
	avoid assuming the various ranges are positive. Note that "mod"
	returns a nonnegative result when the second operand is
	positive. "rem" can return negative values in that case (in
	particular, if the first operand is negative, which it now is).
	* switch-c.adb: Allow switch -gnaten to control the value of
	Nodes_Size_In_Meg.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Remove traling whitespaces.
	* opt.ads (Nodes_Size_In_Meg): New Variable.