ada: Preliminary implementation of structural generic instantiation

ada: Preliminary implementation of structural generic instantiation

It contains the changes to the parser required for the new syntax, as well
as the mechanism to instantiate generics implicitly.  The implementation is
strictly structural, in the sense that if the implicit instantiation cannot
be made structural for semantic dependence reasons, then it is rejected.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst (Structural Generic
	Instantiation): New entry
	* einfo.ads (Is_Link_Once): New flag defined in entities.
	* sinfo.ads (Is_Structural): New flag defined in instantiations.
	* gen_il-fields.ads (Opt_Field_Enum): Add Is_Link_Once and
	Is_Structural.
	* gen_il-gen-gen_entities.adb (Entity_Kind): Add Is_Link_Once
	semantic flag.
	* gen_il-gen-gen_nodes.adb (N_Generic_Instantiation): Move up
	Parent_Spec field and add Is_Structural semantic flag.
	* frontend.adb: Add with clause for Sem_Ch12.
	(Frontend): After analysis is complete and bodies are instantiated,
	call Sem_Ch12.Mark_Link_Once on the declarations of the main unit.
	* par.adb (P_Qualified_Simple_Name): Delete.
	(P_Qualified_Simple_Name_Resync): Likewise
	(P_Exception_Name): New function declaration.
	(P_Label_Name): Likewise.
	(P_Loop_Name): Likewise.
	(P_Generic_Unit_Name): Likewise.
	(P_Library_Unit_Name): Likewise.
	(P_Package_Name): Likewise.
	(P_Parent_Unit_Name): Likewise.
	(P_Subtype_Name): Likewise.
	(P_Subtype_Name_Resync): Likewise.
	* par-ch3.adb (P_Subtype_Mark_Resync): Replace call to
	P_Qualified_Simple_Name_Resync by P_Subtype_Name_Resync.
	(P_Identifier_Declarations): Replace call to
	P_Qualified_Simple_Name_Resync by P_Exception_Name.
	(P_Derived_Type_Def_Or_Private_Ext_Decl): Replace call to
	P_Qualified_Simple_Name by P_Subtype_Name.
	(P_Interface_Type_Definition): Replace calls to
	P_Qualified_Simple_Name by P_Subtype_Name.
	* par-ch4.adb (P_Reduction_Attribute_Reference): Move around and
	change name of parameter.
	(P_Name): Document new grammar rule and make a couple of tweaks.
	(P_Exception_Name): New function.
	(P_Label_Name): Likewise.
	(P_Loop_Name): Likewise.
	(P_Generic_Unit_Name): Likewise.
	(P_Library_Unit_Name): Likewise.
	(P_Package_Name): Likewise.
	(P_Parent_Unit_Name): Likewise.
	(P_Subtype_Name): Likewise.
	(P_Subtype_Name_Resync): Likewise.
	(P_Qualified_Simple_Name): Rename into...
	(P_Simple_Name): ...this.
	(P_Qualified_Simple_Name_Resync): Rename into...
	(P_Simple_Name_Resync): ...this.  Accept left parenthesis and
	dot as name extensions.
	(P_Allocator): Replace call to P_Qualified_Simple_Name_Resync
	by P_Subtype_Name_Resync.
	* par-ch5.adb (P_Goto_Statement): Replace call to
	P_Qualified_Simple_Name by P_Label_Name.
	(Parse_Loop_Flow_Statement): Replace call to
	P_Qualified_Simple_Name by P_Loop_Name.
	* par-ch6.adb (P_Subprogram): Replace call to
	P_Qualified_Simple_Name by P_Generic_Unit_Name.
	* par-ch7.adb (P_Package): Replace calls to
	P_Qualified_Simple_Name by P_Package_Name and P_Generic_Unit_Name.
	* par-ch8.adb (P_Use_Package_Clause): Replace calls to
	P_Qualified_Simple_Name by P_Package_Name.
	* par-ch9.adb (P_Task): Replace call to
	P_Qualified_Simple_Name by P_Subtype_Name.
	(P_Protected): Likewise.
	* par-ch10.adb (P_Context_Clause): Replace call to
	P_Qualified_Simple_Name by P_Library_Unit_Name.
	(P_Subunit): Replace call to P_Qualified_Simple_Name by
	P_Parent_Unit_Name.
	* par-ch12.adb (P_Generic): Replace call to
	P_Qualified_Simple_Name by P_Generic_Unit_Name.
	(P_Formal_Derived_Type_Definition): Replace call to
	P_Qualified_Simple_Name by P_Subtype_Name.
	(P_Formal_Package_Declaration): Replace call to
	P_Qualified_Simple_Name by P_Generic_Unit_Name.
	* sem_ch4.adb: Add with and use clauses for Sem_Ch12.
	(Analyze_Call): Accept implicit instantiations with -gnatX0.
	(Analyze_Indexed_Component_Form): Likewise.
	* sem_ch8.adb (Analyze_Use_Package): Add guard before inserting
	a with clause automatically when there is a use clause.
	(Check_In_Previous_With_Clause): Retrieve original names.
	(Check_Library_Unit_Renaming): Deal with structural instances.
	(End_Use_Type): Minor tweak.
	* sem_ch10.adb (Analyze_With_Clause): Remove useless test and
	call Defining_Entity_Of_Instance.
	* sem_ch12.ads (Build_Structural_Instantiation): New function.
	(Mark_Link_Once): New procedure.
	* sem_ch12.adb: Add with and use clauses for Exp_Dbug.
	(Analyze_Associations): Add support for structural instantiations.
	(Analyze_Package_Instantiation): Likewise.
	(Analyze_Subprogram_Instantiation): Likewise.
	(Analyze_Structural_Associations): New procedure.
	(Need_Subprogram_Instance_Body): Return True for instantiation is
	in the auxiliary declarations of the main unit.
	(Build_Structural_Instantiation): New function.
	(Mark_Link_Once): New procedure.
	* sem_util.ads (Add_Local_Declaration): New procedure.
	(Defining_Entity_Of_Instance): New function.
	* sem_util.adb (Add_Local_Declaration): New procedure.
	(Defining_Entity_Of_Instance): New function.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.