[Ada] Support for Object.Op subprogram-call notation for untagged types

[Ada] Support for Object.Op subprogram-call notation for untagged types

[Pierre-Marie de Rodat]

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This is a prototype implementation of a language extension that allows
calls to primitives of untagged types to be written using prefixed
notation.  That is, the first actual parameter of the call is given as a
prefix using selected_component syntax, and the selector is a primitive
operation of the actual's type). The basic implementation approach is to
employ the Direct_Primitive_Operations field for use by untagged types.
While we could choose to include only the primitives for which a
prefixed call could be made, it seems better to include all primitives,
consistent with the meaning of this list and for uniformity with tagged
types.

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

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Add a
	description of the feature of prefixed-view calls for untagged
	types to the section on pragma Extensions_Allowed.
	* gnat_rm.texi: Regenerate.
	* einfo.ads: Update specification for
	Direct_Primitive_Operations to reflect its use for untagged
	types when Extensions_Allowed is True.
	* gen_il-gen-gen_entities.adb: Allow Direct_Primitive_Operations
	as a field of untagged classes of types by removing the "Pre"
	test of "Is_Tagged_Type (N)", and making that field generally
	available for all types and subtypes by defining it for
	Type_Kind and removing its specification for individual classes
	of types.
	* sem_ch3.adb (Analyze_Full_Type_Declaration): Initialize the
	Direct_Primitive_Operations list when not already set for the
	new (sub)type and its base type (except when Ekind of the type
	is E_Void, which can happen due to errors in cases where
	Derived_Type_Declaration is called and perhaps in other
	situations).
	(Analyze_Subtype_Declaration): Inherit
	Direct_Primitive_Operations list from the base type, for record
	and private cases.
	(Build_Derived_Record_Type): Initialize the
	Direct_Primitive_Operations list for derived record and private
	types.
	(Build_Derived_Type): Initialize the Direct_Primitive_Operations
	list for derived types (and also for their associated base types
	when needed).
	(Process_Full_View): For full types that are untagged record and
	private types, copy the primitive operations of the partial view
	to the primitives list of the full view.
	* sem_ch4.adb (Analyze_Selected_Component): Allow prefixed
	notation for subprogram calls in the case of untagged
	types (when Extensions_Allowed is True). In the case where
	Is_Private_Type (Prefix_Type) is True, call Try_Object_Operation
	when a discriminant selector wasn't found. Also call
	Try_Object_Operation in other type kind cases (when
	Extensions_Allowed is True).
	(Try_Object_Operation.Try_One_Prefixed_Interpretation): Prevent
	early return in the untagged case (when Extensions_Allowed is
	True). Condition main call to Try_Primitive_Operation on the
	type having primitives, and after that, if Prim_Result is False,
	test for case where the prefix type is a named access type with
	primitive operations and in that case call
	Try_Primitive_Operation after temporarily resetting Obj_Type to
	denote the access type (and restore it to the designated type
	after the call)
	(Try_Object_Operation.Valid_First_Argument_Of): Do matching type
	comparison by testing Base_Type (Obj_Type) against
	Base_Type (Typ), rather than against just Typ, to properly
	handle cases where the object prefix has a constrained
	subtype.  (Fixes a bug discovered while working on this
	feature.)
	* sem_ch6.adb
	(New_Overloaded_Entity.Check_For_Primitive_Subprogram): Add a
	primitive of an untagged type to the type's list of primitive
	operations, for both explicit and implicit (derived, so
	Comes_From_Source is False) subprogram declarations. In the case
	where the new primitive overrides an inherited subprogram,
	locate the primitives Elist that references the overridden
	subprogram, and replace that element of the list with the new
	subprogram (done by calling the new procedure
	Add_Or_Replace_Untagged_Primitive on the result type and each
	formal atype).
	(Check_For_Primitive_Subprogram.Add_Or_Replace_Untagged_Primitive):
	New nested procedure to either add or replace an untagged
	primitive subprogram in a given type's list of primitive
	operations (replacement happens in case where the new subprogram
	overrides a primitive of the type).
	* sem_ch7.adb (New_Private_Type): When Extensions_Allowed is
	True, initialize the Direct_Primitive_Operations list of a
	private type to New_Elmt_List in the case of untagged types.
	* sem_ch8.adb (Find_Selected_Component): In the case where the
	prefix is an entity name, relax condition that tests
	Has_Components so that Analyze_Selected_Component will also be
	called when Extensions_Allowed is True and the prefix type is
	any type.

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