[Ada] AI-443, 240, 195, 317

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk.

On going work to support various AIs, in particular: AI-443, AI-240, AI-195 and
AI-317.

In particular, this change adds an Entity field to attribute definition clause
nodes.  It is used only for stream attributes definition clauses. In this case,
it denotes a (possibly dummy) subprogram entity that is conceptually declared at
the point of the clause. Thus the visibility of the attribute definition
clause (in the sense of 8.3(23) as amended by AI-195) can be checked by
testing the visibility of that subprogram.

2006-10-31  Robert Dewar  <dewar@adacore.com>
	    Ed Schonberg  <schonberg@adacore.com>
	    Bob Duff  <duff@adacore.com>

	* sinfo.ads, sinfo.adb (Set_Synchronized_Present,
	Synchronized_Present): Add Formal_Derived_Type_Definition and
	Private_Extension_Declaration to the list of assertions.
	(Is_Entry_Barrier_Function): New flag
	(Has_Self_Reference): New flag on aggregates, to indicate that they
	contain a reference to the enclosing type, inserted through a default
	initialization.
	(Next_Rep_Item): Move from Node4 to Node5.
	(Entity): Add this field for N_Attribute_Definition_Clause.
	(Comes_From_Extended_Return_Statement): New flag on N_Return_Statement
	(N_Return_Object_Declaration): Remove this node kind. We now use
	N_Object_Declaration instead.
	(Actual_Designated_Subtype): Move to a different place to make room in
	N_Extended_Return_Statement.
	(Procedure_To_Call): Move to a different place to make room in
	N_Extended_Return_Statement.
	(Return_Type): Removed this field to make room in return statements
	(both kinds).
	(Return_Statement_Entity): New field in return statements, in part to
	replace Return_Type, and in part to support the fact that return
	statements are now pushed on the scope stack during semantic analysis.
	(Return_Object_Declarations): New field to support extended return
	statements.
	(N_Extended_Return_Statement): New node for extended_return_statement
	nonterminal.
	(N_Return_Object_Declaration): New node for part of
	extended_return_statement nonterminal. Needed because all the necessary
	fields won't fit in N_Extended_Return_Statement.
	Generic_associations now carry the Box_Present flag, to indicate a
	default for an actual in a partially parametrized formal package.

        * snames.h, snames.ads, snames.adb: Add definition for Validity_Check
	(Preset_Names): Add entries for Priority_Specific_Dispatching pragma
	and for the new predefined dispatching policies: EDF_Across_Priorities,
	Non_Preemptive_Within_Priorities, and Round_Robin_Within_Priorities.
	Introduce new name Stub_Type for implementation defined attribute.
	Add pragma Preelaborable_Initialization
	Add entry for Priority attribute.
	Add Pragma_Wide_Character_Encoding
	(Get_Convention_Name): Given a convention id, this function returns the
	corresponding name id from the names table.

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