ada: Elide the copy in extended returns for nonlimited by-reference types

ada: Elide the copy in extended returns for nonlimited by-reference types

This implements elision of the copy operation for extended return statements
in the case of nonlimited by-reference types (the copy operation is already
elided for limited types by the front-end and nonlimited non-by-reference
types by the code generator), which comprise controlled and tagged types.

The implementation partly reuses the machinery implemented for limited types
(the build-in-place machinery) to allocate the return object directly on the
primary or the secondary stack, depending on whether the result type of the
function is constrained or not.

This requires further special-casing for the allocators generated by this
machinery as well as an adjustment to the implementation of a specific case
of string concatenation.

gcc/ada/

	* einfo.ads (Actual_Subtype): Document additional usage.
	* exp_aggr.adb (Expand_Array_Aggregate): Replace test on
	Is_Build_In_Place_Return_Object with Is_Special_Return_Object.
	* exp_ch3.adb (Expand_N_Object_Declaration): Factor out parts of the
	processing done for build-in-place return objects and reuse them to
	implement a similar processing for specific return objects.
	* exp_ch4.adb (Expand_Allocator_Expression): Do not generate a tag
	assignment or an adjustment if the allocator was made for a special
	return object.
	(Expand_Concatenate): If the result is allocated on the secondary
	stack, use an unconstrained allocation.
	* exp_ch6.ads (Apply_CW_Accessibility_Check): New declaration.
	(Is_By_Reference_Return_Object): Likewise.
	(Is_Secondary_Stack_Return_Object): Likewise.
	(Is_Special_Return_Object): Likewise.
	* exp_ch6.adb (Expand_Ctrl_Function_Call): Do not bail out for the
	expression in the declaration of a special return object.
	(Expand_N_Extended_Return_Statement): Add missing guard and move
	the class-wide accessibility check to Expand_N_Object_Declaration.
	(Expand_Simple_Function_Return): Delete obsolete commentary.
	Skip the special processing for types that require finalization or
	are returned on the secondary stack if the return originally comes
	from an extended return statement.  Add missing Constant_Present.
	(Is_By_Reference_Return_Object): New predicate.
	(Is_Secondary_Stack_Return_Object): Likewise.
	(Is_Special_Return_Object): Likewise.
	* exp_util.adb (Is_Related_To_Func_Return): Also return true if the
	parent of the expression is the renaming declaration generated for
	the expansion of a return object.
	* gen_il-fields.ads (Opt_Field_Enum): Replace Alloc_For_BIP_Return
	with For_Special_Return_Object.
	* gen_il-gen-gen_nodes.adb (N_Allocator): Likewise.
	* gen_il-internals.adb (Image): Remove Alloc_For_BIP_Return.
	* sem_ch3.adb (Check_Return_Subtype_Indication): New procedure
	moved from sem_ch6.adb.
	(Analyze_Object_Declaration): Call it on a return object.
	* sem_ch4.adb: Add with and use clauses for Rtsfind.
	(Analyze_Allocator): Test For_Special_Return_Object to skip checks
	for allocators made for special return objects.
	Do not report restriction violations for the return stack pool.
	* sem_ch5.adb (Analyze_Assignment.Set_Assignment_Type): Return the
	Actual_Subtype for return objects that live on the secondary stack.
	* sem_ch6.adb (Check_Return_Subtype_Indication): Move procedure to
	sem_ch3.adb.
	(Analyze_Function_Return): Do not call above procedure.
	* sem_res.adb (Resolve_Allocator): Replace Alloc_For_BIP_Return
	with For_Special_Return_Object.
	* sinfo.ads: Likewise.
	* treepr.adb (Image): Remove Alloc_For_BIP_Return.
	* gcc-interface/trans.cc (gnat_to_gnu): Do not convert to the result
	type in the unconstrained array type case if the parent is a simple
	return statement.