ada: Initial support for Extended Access types

ada: Initial support for Extended Access types

This change introduces the support for the new GNAT specific
Extended_Access aspect for access to unconstrained array type :

  type Ext_Access is access all Some_Array_Type with Extended_Access;

 This new kind of access type does not use the existing "fat" layout with
 a record of two pointers: one to the actual data, one to a record with the
 bounds of the array. Instead, it removes the second indirection and extends
 the record to contain the pointer to the actual data followed by the bounds.

This mainly allows the following features:
- have access to slice of array
- easier interface when allocation is done in a foreign language

gcc/ada/ChangeLog:

	* gcc-interface/ada-tree.h (TYPE_EXTENDED_POINTER_P): New.
	(TYPE_IS_EXTENDED_POINTER_P): New.
	(TYPE_EXTENDED_UNCONSTRAINED_ARRAY): New.
	(SET_TYPE_EXTENDED_UNCONSTRAINED_ARRAY): New.
	(TYPE_DUMMY_EXT_POINTER_TO): New.
	(SET_TYPE_DUMMY_EXT_POINTER_TO): New.
	* gcc-interface/decl.cc (get_extended_unconstrained_array): New.
	(gnat_to_gnu_entity): Handle extended access type.
	(get_unpadded_extended_type): New.
	(gnat_to_gnu_component_type): Handle extended access type.
	(build_template_type): New.
	(gnat_to_gnu_field): Handle extended access type.
	(validate_size): Likewise.
	(set_rm_size): Likewise.
	(copy_and_substitute_in_layout): Likewise.
	(rm_size): Likewise.
	* gcc-interface/gigi.h (get_unpadded_extended_type): New.
	(build_template_type): New.
	(build_dummy_unc_pointer_types_ext): New.
	(finish_extended_pointer_type): New.
	(build_unc_object_type_from_ptr): Rename first parameter.
	* gcc-interface/misc.cc (gnat_print_type): Handle extended access type.
	* gcc-interface/trans.cc (Identifier_to_gnu): Likewise.
	(Attribute_to_gnu): Likewise.
	(gnat_to_gnu): Likewise.
	* gcc-interface/utils.cc (convert_to_fat_pointer): Assert if converting an
	extended pointer.
	(build_dummy_unc_pointer_types_ext): New.
	(finish_extended_pointer_type): New.
	(finish_record_type): Handle extended access type.
	(build_unc_object_type_from_ptr): Likewise.
	(convert_to_extended_pointer): New.
	(convert): Handle extended access type.
	(gnat_pushdecl): Likewise.
	(maybe_pad_type): Likewise.
	* gcc-interface/utils2.cc (build_unary_op): Likewise.
	(build_binary_op): Likewise.
	(build_allocator): Likewise.
	(gnat_save_expr): Likewise.
	(gnat_protect_expr): Likewise.
	(gnat_stabilize_reference_1): Likewise.